It is well established that electrons and protons whirl in every atom; packs of atoms assemble in every molecule; societies of molecules create cells; and your body is a metropolis of cells. So, are we all essentially the same? No. Medawar’s story of graft rejection showed that my body can tell apart my cells from yours. Recall that his patients’ bodies could only accept skin grafted from elsewhere on their own bodies; skin taken from the bodies of others, even relatives, was rejected. How can this be? What molecular substance gives each of us our individuality and how could our bodies distinguish it? And this is where Frank Macfarlane Burnet moves things forward – by asking: how does our body know its tissues and cells as its own? Or, put another way, how does the human body discriminate self from non-self?
Burnet was an introvert; ‘a fairly humourless dry old stick who wouldn’t let his hair down – the opposite of Medawar’, Leslie Brent recalls.1 But he is also one of the greatest thinkers there has ever been in human biology. In 1937, aged thirty-eight, Burnet formulated the idea that discriminating between what’s you and what’s not you is the immune system’s raison d’être, that recognizing and destroying substances that are non-self is precisely what the immune system must do. And from this Burnet realized that the problem of how our body recognizes disease is part and parcel of understanding how our body knows its own cells and tissues.
This huge step forward in understanding how our immune system works descends directly from the simple fact that disease can be caused by germs. Beyond its obvious practical importance, knowledge of germs helped us to understand that disease is caused by something outside of us, something non-self. Although we all now know that germs cause disease, this fact took millennia to establish. Indeed, the history of how humans have struggled to understand disease is important in illustrating how revolutionary Burnet’s ideas really were.
The Greek philosopher and physician Hippocrates, born around 460 BCE, is considered the first to have suggested that disease is not a direct act of God, or an outcome from some superstitious belief, but that instead it has a natural cause. Greek physicians, and later the Romans, took as fact that disease came about from an excess or deficiency of one of four ‘humours’ – black bile, yellow bile, phlegm and blood – each of which had to be present at the right levels for us to be healthy. This view endured, essentially unchanged, for two millennia.2
A description of disease is not mere semantics: past misunderstandings have brought out the worst in human behaviour. When the Black Death arrived in Europe in 1347, a true understanding of disease was still centuries away, and the beliefs of the age had grave consequences. Estimates put deaths caused by the plague at anywhere between 75 and 200 million, slashing Europe’s population by at least a third, and possibly half. It would return in waves – though never again to such catastrophic effect – for the next 400 years. Inevitably, crowded cities were worst hit: half the populations of Paris and London perished. Chroniclers of the time said the living were scarcely able to bury the dead; that the devastation seemed more final than Noah’s flood.3 Doctors had only opinions, not facts, to explain what was going on. Most people believed that humanity was being punished by God, while astrologers asserted that the horror was caused by an alignment of planets Mars, Saturn and Jupiter (even though this doesn’t seem able to explain why only some people succumbed to the plague).
A belief that the plague was caused by sins against God twisted into a desire to kill the enemies of Christ. One common belief was that the Black Death was spread by Jews and other non-Christians. Jews were accused of poisoning water wells in an attack against Christianity, and often confessed to this under torture. In vengeance, thousands were murdered in cities across France, Austria and Germany. The sentiment helped seed the following century’s Spanish Inquisition. A lack of understanding about the nature of disease played a role in allowing European leaders to force religious conversion and burn people at the stake. The painful irony is it that a contemporary understanding of disease reveals that human genetic variation is central to our immune defences.
A modern view of disease begins in the nineteenth century, the giants of the era being Charles Darwin and the French microbiologist Louis Pasteur. The two legends never met face-to-face, alas, though it would have been possible. Today, Pasteur gets his name onto almost every packet of cheese, while Darwin is revered, sometimes cursed, for his supposed slaying of God. Pasteur first showed that living cells were essential for making wine and then that a similar budding and multiplying of cells occurred in soured milk. At the time, it was hotly disputed whether fermentation was some kind of mechanical breakdown of chemicals or a biological process. Pasteur clarified that minuscule living organisms, unseen by the human eye, were at the heart of these phenomena. But his brilliance was in realizing that we, too, must be exposed to this new-found world of invisible organisms. Since unseen microbes can cause dramatic changes to the nature of things – as in fermentation – he postulated that these unseen microbes might also underlie human disease. Many thought this a ridiculous idea: how could something so small that it can’t be seen kill something so much more powerful like us?
Pasteur’s ideas about microscopic organisms highlighted a major problem: at the time, nobody knew where minuscule living organisms came from. Could minute life-forms arise from spontaneous chemical reactions when milk goes sour, or when maggots appear in rotting meat, or does life really only ever arise from pre-existing life? For the prestigious French Académie des Sciences, this was the most pressing issue of the day. Pasteur settled the debate with an ingenious simple experiment.
He took a glass flask and shaped its neck into a thin tube bent to an s-shaped curve. To this so-called swan-neck flask he added a clear broth, similar to a soup base, which had been heated to kill off all living things. Although the broth was exposed to the air through the s-shaped neck, nothing would grow in the liquid – microbes and dust particles from the air would collect in the curve of the flask’s neck and not reach the broth. But after Pasteur then broke off the curved neck, the broth would turn cloudy – things now started to grow. Microbes had fallen into the broth from dust in the air. So, life does not spontaneously arise in the broth, it falls in from the air. But another, more subtle implication was that minute organisms are all around us.
That such minute organisms can cause disease in humans was finally established in 1876 by the German scientist and medical doctor Robert Koch, son of a mining engineer. Koch set up a makeshift laboratory in his four-room flat while working as the district medical officer in Wollstein, Western Poland, isolated from libraries and other scientists and without financial support for research, simply using equipment he purchased himself – apart from his microscope, which was a present from his wife. By day he saw his medical patients; out of hours he worked on mice, infecting them with anthrax bacteria that he obtained from the spleens of dead farm animals.
It was already known that organs or blood from an infected animal could pass on the disease. But one of Koch’s brilliant experiments was to culture some of the rod-shaped anthrax bacteria in the fluid from an ox’s eye, and to demonstrate that these cultured, isolated bacteria could still give mice the disease. In this way Koch established, once and for all, that bacteria can cause disease. In fact, we now know there are about 5 × 1032 bacteria on earth. It no longer seems ridiculous that minuscule unseen germs could harm us. Now the more astonishing thing is that our immune system is, more often than not, actually able to protect us.
Koch’s and Pasteur’s discoveries complement each other perfectly but personally they were arch-enemies. For much of their careers, they fired off at each other vicious patriotic claims for their own discoveries, mirroring the Franco-German political disputes of the time.4Koch, younger by twenty years, suggested that Pasteur could not obtain microbes as pure as he could, and that Pasteur’s experiments were usually meaningless. At a meeting in Geneva in 1882, Pasteur, by then aged sixty, directed a barbed observation at Koch, who was seated in the front row. Describing his latest experiments with chicken cholera, which showed that the disease-causing bacteria could be attenuated and used as a vaccine, Pasteur then noted, ‘However blazingly clear the demonstrated truth, it has not always had the privilege of being easily accepted.’ Just to make absolutely clear who he was talking about, he continued: ‘Dr Koch, who finds nothing remarkable in this experiment . . . does not believe that I operated as I said I did, with eighty chickens . . . because that would have cost too much money.’ Sitting with his students, Koch listened unmoved to Pasteur’s nationalist punchline: ‘But in view of establishing this great fact . . . my government allowed me not to worry about the expense.’5
The following year, an editorial in the Boston Medical and Surgical Journal wrote about the debacle with a timeless wisdom that can be transposed to any number of disputes:
It is to be regretted that abstract questions of scientific truth or error cannot be divorced from the personalities of discoverers and wrangling over priority, and that such anger should possess celestial minds. The expanse of the unknown is broad enough for all voyagers to pursue their way without collision.6
But perhaps these words are naive. Pioneers in science, or anything else, must be strong-willed enough to travel in a new direction and thick-skinned enough to withstand criticism from guardians of the prevailing dogma. A level of inner confidence that gets very close to arrogance is often of benefit to any trailblazer; self-belief is as critical as talent.
To relate to this kind of almost stereotyped conflict between scientists it’s important to remember that, while artists are able to delight in their individual output being individual, scientists never really produce anything unique. They can only be first in uncovering information that otherwise would have been discovered by somebody else later. In the end both Pasteur and Koch, as well as many others, contributed to the discovery that germs cause disease. Koch won the Nobel Prize in 1905, but Pasteur had died six years before the first Nobel Prizes were awarded. Both have major institutes named after them today.
The concept of germs is so deeply implanted in us today that it takes effort to appreciate that the idea that so small a thing could be so harmful was initially thought ridiculous. It had to be explicitly proven that disease was not caused by the wrath of evil spirits, or an imbalance of black bile, yellow bile, phlegm and blood, or a poisonous vapour from decaying matter (as in the so-called miasma theory of the Middle Ages). Distinct diseases do have different origins, but many are caused by minuscule microbes, and realizing this is undoubtedly one of the greatest triumphs of the second millennium.
Sanitation and hygiene, as well as almost all of modern medicine, builds on this basic premise. Indeed, in Life magazine’s list of the top 100 most important events in the last millennium, the discovery of germs ranked sixth. Gutenberg’s printing of the Bible came out top, but the existence of germs beats vaccination (thirteenth), evolution (fifteenth), the telephone (twentieth), penicillin (twenty-second), landing on the moon (thirty-third) and the structure of DNA (seventy-sixth).7 Such a list is highly subjective – the ‘discovery’ of Coca-Cola was a surprising choice, to say the least, as the eighty-second most important event in a thousand years – but unquestionably, the sixth choice was good: nothing has done more for our well-being than the epochal discovery of germs. This was also an essential first step towards the important idea that our immune system can defend us by discriminating between our own cells and tissues, i.e. ‘self’, and every other thing out there, i.e. ‘non-self’.
Of course, the language of self and non-self has connotations far beyond our immune system. Many philosophers and religious scholars have discussed the meaning of self as a metaphysical concept as well as in terms of our physical body. Buddha, for instance, discusses self and non-self a lot. Buddha refers to non-self, or not-self, as things perceived by our senses that we must not cling to, while self is the part of us which seeks pleasure, lusts after vanity, brings about envy and gives rise to hatred. The very existence of self, Buddha teaches, is nothing more than an illusion, and we must strive in our lives to be free of this mirage. Such holistic and spiritual interpretations of ‘self’ are simply from different realms of thinking to the molecular description of ‘self’ that immunology provides. Using the language of ‘self’ and ‘non-self’ in describing ideas about the molecules that comprise our bodies is provocative and yet, at some level, individuality surely is constructed from our constituent chemistry.
Burnet started using the terminology of ‘self’ and ‘non-self’ in 1940. Comparatively isolated from any international hub of scientific activity like Medawar’s base in war-torn London, he first used the terms in a loose metaphorical sense, but by 1949 he and his Australian colleague Frank Fenner, fifteen years younger than Burnet, set out their view clearly – in what they called the self-marker hypothesis – that the human immune system works by discriminating its own self from non-self.8
Fenner humbly maintained that in this work he was merely a junior assistant checking a few facts, that ‘Burnet was responsible for all the interpretation and speculation’.9 In fact, Fenner went on to publish over 300 scientific papers and he played a considerable role in the eradication of smallpox.10 And throughout his stellar career, he always kept a photo of Burnet on his desk.11
Quite different from Medawar, Burnet was not driven to solve the clinically important problem of transplantation. Burnet never experienced the shock of pacing a war wounds hospital as Medawar did, and Burnet never collected clinical data himself. Though he had previously studied viruses and had made important experimental discoveries regarding how influenza spread, what drove him was the desire to understand what happened during an immune response: in fact, his burning ambition was to discover a grand unified theory of immunology.
In this respect, Burnet was following in giant footsteps. He was twenty-two when Albert Einstein won the Nobel Prize, and Richard Feynman, a dominant force in physics, was a contemporary of him and Medawar. Both Einstein and Feynman were obsessed with the quest for fundamental laws that unify the different forces in nature. That way of thinking continues today – Stephen Hawking and many others freely talk of our search for a grand unified theory of everything – and Burnet was of the same ilk.
As with his hero Charles Darwin a century earlier, collecting beetles – a vivid portrait of biological diversity – sowed the seeds of Burnet’s tenacious search for generalizations and underlying principles. He made copious notes on unusual beetle behaviours and filled sketch books with drawings of beetle legs and antennae.12 Later in life, after dinners with his family, he would often read the current issues of the scientific journals Nature and Science and keep copious records of what he read on small cards.13
It is likely that he found this solace in collecting and organizing, at least in part, because he never established a deep relationship with his parents.14 His mother was preoccupied with caring for his mentally disabled elder sister Doris and as a result became very reclusive. His father spent little time with the family, preferring to be out with friends, playing golf or trout fishing, and Burnet later recalled that, even as young as eight, he disapproved of some of his father’s deals as the local bank manager.15 Burnet’s sister’s disability, resulting from complications at birth, was not allowed to be discussed outside the family, and friends were discouraged from coming over to play. Perhaps this contributed to him growing up to be shy and introspective – ‘always something of a solitary’, he said of himself.16 By age seven, Burnet had already won a prize for academic achievement at school and he later graduated as the second-top student in Medicine at the University of Melbourne.17 He loved the wealth of the Melbourne Public Library – especially all the ‘knowledge enshrined there about the anatomy of beetles’.18
On the evening of 21 October 1921, he heard his father was seriously ill back at the family home in Terang, a small town of around 2,000 residents just over 200 kilometres south-west of Melbourne. The next day, Burnet rushed back to Terang by train with the intention of laying to rest the strain in their distant relationship, but it was too late.
One thing Burnet learned from his father was patriotism; in part a reaction against Australia being seen as an English colony. It was always important for Burnet to prove that Australian science could stand up to be as good as science anywhere else in the world.19 In 1944, based on his early success studying viruses, he was offered a lucrative position at Harvard University, which offered a research environment that far surpassed Melbourne, but he turned it down because he thought that his children should grow up in Australia.20
Burnet had been especially shy with women, and danced with a girl for the first time aged twenty-four. Soon after, he met his future wife Linda through an arranged introduction. Like Jean Medawar, Linda accepted his need to work hard and to be left alone often. Despite eventually being thrust into the limelight as a Nobel laureate and director of a prestigious institute, he always kept his family life very private. In his autobiography, published in 1968, he wrote of Linda that ‘beyond recording that we were married on 10 July 1928, I shall say nothing more directly about her’.21 The eldest of their three children, Elizabeth Dexter, recalled in 2011 that Linda would commonly ‘fob off anyone that threatened to disrupt his work’:22 ‘He could never say no, but she could!’23 Just like Medawar, Burnet was shielded from distractions by his wife, who saw her role defined by having a husband capable of immensely important discoveries.
Burnet, like Medawar again, was especially influenced by Ray Owen’s experiments. Owen, we recall, discovered that cattle twins in a non-identical pair are tolerant to cells from each other, presumably because of a shared placenta.24 In fact, the seminal importance of Owen’s work was little recognized until Burnet and Fenner highlighted it in their 1949 publication – Medawar, for example, wasn’t even aware of Owen’s work until he read about it in Burnet and Fenner’s paper.25
Building on Owen’s discovery, Burnet speculated that the twins’ tolerance for each other must have developed by the calves being exposed to the other’s cells when foetuses. From this, he went on to suggest that the human immune system must also learn to recognize our own cells and tissues when foetuses or in early childhood. They didn’t understand how this could work in any detail but it just seemed to make sense that the immune system would learn what our body is made of at an early age so that it is then ready to attack anything else.
Burnet had no proof of this hypothesis, and in 1949 could only conclude that ‘it remains to be seen whether this concept is of value’.26 It was a few years later that Medawar’s skin grafting experiments, essentially those three and a half pages reported in 1953, showed these ideas to be right: that the immune system would indeed become tolerant to any cells or tissues present early on in an animal’s life.
Although this won Burnet and Medawar the Nobel Prize in 1960, nobody had any idea how this really worked: how did the immune system learn to recognize the body’s own cells and tissues early in life? Indeed, Burnet considered this Nobel-Prize-winning achievement as being ‘essentially only a way-station on the road to the broader conception’ of how the immune system worked.27 He was right: his next theory has claim to being far more important than that which won him the Nobel Prize.
The focus of Burnet’s thinking, and that of many of his contemporaries, was antibodies. Discovered in 1890, antibodies are soluble proteins found in blood that stick to and neutralize all kinds of germs and potentially dangerous molecules. The key problem lay in understanding how such antibodies could recognize so many different kinds of germs, while seemingly not triggering an attack on our own cells or tissue. And here’s the really big mystery: by the mid-1950s, while chemists over the world were starting to synthesize new molecules that had never existed before, biologists found that the human body is able to make antibodies that recognize and stick to these brand-new molecules. It’s one thing to try and work out how antibodies could recognize specific germs, but here was evidence that in fact antibodies can recognize anything, even brand-new molecules that have never existed in the universe before. How could this work? Everyone agreed that this was the greatest problem in our understanding of how our immune system worked: how could antibodies react to a potentially limitless number of ‘non-self’ molecules but still not mount an attack on ‘self’ cells and tissues?
The prevalent view, led by the two-time Nobel-Prize-winning American biochemist Linus Pauling, was that antibodies could mould into any shape to fit around foreign molecules and trigger their destruction. This was the so-called ‘instructional theory’: a generic antibody is instructed by the foreign molecule to fit around it. London-born Danish scientist Niels Jerne, who went on to win the Nobel Prize for Medicine in 1984, didn’t like this idea at all.
Jerne was a late developer in science. He grew up in the Netherlands and studied physics at the University of Leiden, but he then spent thirteen years in various occupations before deciding to return to academic study, this time medicine, and eventually he obtained a PhD in Copenhagen in 1951, aged nearly forty. After his PhD, while working in Copenhagen at the Danish National Serum Institute, a government research institute for infectious diseases, Jerne thought about the antibody problem. For much of his life before that time he had considered himself in a ‘dark middle age’28 – certainly not thinking about immunology. Even so, it is telling that, throughout his life, he saved nearly all his correspondence and manuscripts in anticipation that one day they would be important – confident that his life would be of widespread interest in the end.
Prior to thinking about antibodies, Jerne had many difficulties in his life.29 Sexually, he was keen on sadomasochism, but it is not clear if he shared this tendency with his wife Tjek. They bonded through their passion to live free from the shackles of social convention; but both had extra-marital affairs and their relationship suffered as a result. Tjek was a very successful artist in Copenhagen and she struggled with everyday difficulties raising their two children while Jerne was often absent, working in his father’s Danish bacon business. In 1945, Tjek committed suicide. Jerne struggled with guilt: he had threatened Tjek with divorce. Immersion in science – and specifically antibodies – may have been his escape.30
Now that he had found something to focus his mind on, Jerne felt that the implication that any molecule at all could instruct cells how to make a well-suited antibody just seemed ‘odd’.31 There were also some more specific issues that worried Jerne, such as how an antibody could know which molecule it should fold around among all the components of a cell – why would antibodies only fold around non-self molecules?
Then one day, some nine years after his wife’s suicide and four years after the publication of Burnet’s paper, Jerne had a moment of inspiration on his way home from work. During his brisk twenty-minute walk, he formulated a new theory for how self and non-self discrimination worked.32 He suddenly thought to himself that maybe all different shapes of antibody exist beforehand – a whole collection of differently shaped antibodies just circulating in the blood before any germ has been seen – so that any particular foreign molecule will be recognized by at least one of the pre-existing shapes of antibody. This would become known as ‘selectionist theory’, because there would be an antibody that could be selected from the pool of all the differently shaped antibodies, able to stick to the foreign molecule, leading to the molecule’s destruction. Immediately, Jerne thought his theory had to be correct.33
James Watson, the scientist celebrated for his work with Francis Crick on the double helix shape of DNA, had come as a student in 1950 to work with Jerne at the Danish National Serum Institute, and the two had remained friends ever since. Watson listened carefully to Jerne’s idea about antibodies and decided that it stank.34 Pauling similarly dismissed it without hesitation.35 Both thought the idea wrong because their practical experience of studying the shapes of molecules suggested to them that there simply couldn’t be such an enormous pool of differently shaped antibodies pre-existing in our blood.
Burnet, however – lacking practical experience in examining the shapes of molecules – responded differently. He was immediately excited by the idea and – theoretically – realized it could be correct. He considered Jerne ‘the most intelligent immunologist alive’36 and, thinking deeply about his theory, came up with a crucial modification, which Jerne described as another ‘guess’. Burnet presented his ideas concisely in two pages in his paper ‘A modification of Jerne’s theory of antibody production using the concept of clonal selection’, published in the Australian Journal of Science in 1957, when he was aged fifty-seven – somewhat older than when scientists are usually thought to have their best, ground-breaking idea.37
Burnet’s ‘modification’ to Jerne’s theory was hugely important. It single-handedly transformed the reputation of both men – this was still three years before Burnet won the Nobel Prize with Medawar – and changed Jerne’s original idea from one that ‘stank’ to one that has become a cornerstone in understanding how everything works in the immune system. In general, scientific papers do well to be remembered a few years after publication, but in this case, on the fiftieth anniversary of Burnet’s paper, today’s leading specialist journal Nature Immunology remarked on its exceptional significance: ‘rarely has a field as large and influential been gathered together and encapsulated in so spare a form’.38
Burnet’s modification was that the focus should be on the cells that make antibodies rather than the antibodies themselves.39 Burnet speculated that one cell makes one particular shape of antibody and that all our antibody-making immune cells together make an unimaginably vast repertoire of 10 billion antibodies, each having a slightly different shape. So, for any particular non-self molecule that enters the body, at least one immune cell will make an antibody that is the right shape to stick to that particular molecule. When a cell sees a molecule that its antibodies can stick to, it multiplies, and lots of clones of the initial cell can then secrete the right antibody in bulk, efficiently neutralizing the dangerous molecule or germ. Burnet named the idea Clonal Selection Theory.40
Around the same time, American immunologist David Talmage, from the University of Colorado, published some related ideas. Yet, in a controversy that endures to this day, Burnet’s legacy has put Talmage’s contribution in the shade. It is a situation comparable to that of the Welsh-born naturalist Alfred Russel Wallace and his contemporary Charles Darwin, a century earlier. In June 1958, Wallace sent Darwin a twenty-odd-page letter detailing his ideas on how species can diverge as a result of environmental pressures. Darwin had independently been accumulating evidence for this idea over decades and the letter spurred him on to complete his book-length treatise on the idea, On the Origin of Species, published on 24 November 1859. Likewise, in 1956, Talmage sent Burnet an advance copy of his paper with his own ideas similar to Burnet’s Clonal Selection Theory. Talmage had been inspired by Burnet and Fenner’s 1949 paper describing the immune system as discriminating self and non-self.41 Burnet, however, had already come to the same conclusions independently, and – again reminiscent of Darwin’s primacy over Wallace – there is no doubt that he had explored the implications of the theory more completely.42 In any case, almost everyone rejected the idea at first – Talmage puts it down to a general scepticism that all new ideas are judged by.43
The dominant argument against Burnet and Talmage was: why should immune cells make an enormous array of pre-existing antibodies able to recognize all manner of non-self molecules, most of which would never be used? It seemed wasteful and counter-productive. Yet for Burnet the idea was self-evident: simply put, it was Darwinian selection applied to cells inside our bodies. Indeed, Burnet viewed the human body like an ecosystem – a dynamic place where cells interact and can multiply or die. From this perspective, he envisaged that the cells in our immune system best suited to fight a particular germ could be activated to multiply and become a greater fraction of the total population of antibody-secreting cells.
In 1957, Gustav Nossal, a confident twenty-six-year-old Austrian medic working in the Hall Institute, Melbourne, suggested to the fifty-eight-year-old Burnet that he could easily disprove his ‘pretty crazy theory’ of clonal selection by showing that a single cell could actually make more than just one shape of antibody.44 Nossal had just recently arrived in Melbourne, with his wife and new baby, to study for a PhD, having studied medicine in Sydney. To Nossal’s surprise, Burnet was both excited and encouraging, and this was the beginning of a long collaboration between them. Over the years, Nossal learned that it worked best to never bluntly disagree with Burnet, and it paid great dividends to always subtly acknowledge his primacy.45
In 1957, Burnet didn’t only encourage Nossal in his idea but also found him help with the experiment, in the form of US scientist Josh Lederberg. Lederberg, son of an orthodox Rabbi, was thirty-two, just six years older than Nossal, but he was already renowned for his pioneering research in the genetics of bacteria and was soon to win the Nobel Prize in 1958. Lederberg arrived at the Hall Institute in 1957 for a three-month sabbatical to work with Burnet, who suggested that he help Nossal. Excited by Nossal’s idea, Lederberg taught him how to use a micromanipulator, a tool that allows you to move and manipulate objects under a microscope. The micromanipulator, it turned out, was the key to isolating individual antibody-secreting cells in separate liquid drops – a process crucial to the experiment Nossal was about to carry out.
In the end, the actual breakthrough happened late in 1957, when Lederberg had already returned to the US. Nossal injected rats with two different strains of bacteria. He then isolated the antibody-secreting cells from the infected rats in droplets and watched when he added bacteria. His aim was to detect whether a single cell could stop the movement of one or both types of bacteria. What he found was that, while many of the single cells were able to stop one of the strains of bacteria moving, none of the single cell droplets had antibodies that could stop both.
This was a huge step. It showed that a single cell was capable of neutralizing only one type of bacteria – so a single cell must make just one shape of antibody.46 This was the first experiment that really helped Burnet gain acceptance for his Clonal Selection Theory – the theory that an individual cell will be activated to multiply when that cell is the one that makes the right-shaped antibody to neutralize a problematic germ.
Subsequently, Nossal became as close to the self-contained, distant Burnet as anybody did in the institute: a friendship based on mutual intellectual respect. And Burnet nominated the precocious Nossal his successor as head of the Hall Institute, a post he assumed in 1965 at the age of thirty-five. Nossal was outgoing, entrepreneurial, a gifted speaker and a very successful leader. While the institute had an annual income of 350,000 Australian dollars when Burnet had left in 1965, Nossal had increased this to over 25 million by 1992.47
After he retired as director of the Hall Institute, Burnet became much more outspoken on broad issues such as ageing, the limits of medicine and the future of humanity.48 He wrote books that didn’t simply aim at making science accessible, but set out to answer big philosophical questions. To the surprise of many, given his constant and ongoing fear of speaking in public – which would often bring on migraines before interviews – he became Australia’s leading scientific spokesperson.49 In his penultimate book, published in 1978, Burnet’s views were particularly uncompromising. While acknowledging the anathema of Hitler’s racist policies, he wrote sympathetically about the underlying principles of eugenics. He spoke coolly and dispassionately about infanticide and euthanasia. His motive was a ‘compassion to those individuals predestined to intolerable life’ and not ‘the idea of producing a better human species’.50
In 1969, a few years after Burnet retired from leading the Hall Institute, his wife Linda was diagnosed with lymphoid leukaemia, and from then on Burnet refused any offers to lecture abroad. After her death in November 1973, Burnet, devastated and isolated, found solace once again in collecting beetles, and he secretly wrote letters addressed to Linda every Sunday evening.51 In 1976, he married a second time and subsequently expanded his public engagements.
Burnet died of cancer on 31 August 1985, comfortable that his discoveries would survive him. A staunch atheist, he had no time for any religious opinion about what happens when you die. Talmage wrote that Burnet was ‘a dominant figure in immunology and medical science for half a century’,52 and Leslie Brent remarked that he was ‘one of the deepest thinkers immunology has produced.’53 Today, Burnet is remembered for Clonal Selection Theory far more than his Nobel-Prize-winning understanding of Medawar’s experiments in acquired tolerance. But the details of how Burnet’s theories really worked were left to others to sort out. For example, he didn’t really know how each cell could make a differently shaped antibody.
At the time that Burnet formulated his theory, Crick, Watson and others had already worked out that a single gene encoded the instruction to make a single protein. Nossal’s experiment showed that one cell made one specific shape of antibody, which backed up Burnet’s idea, but raised the problem of how. That is, how could antibody-making cells each make differently shaped antibodies? The number of different shapes of antibodies in the human immune system – estimated to be 10–100 billion – far outstrips the number of genes we have – 25,000. So, how could each cell make a differently shaped antibody when it’s impossible that each variation of antibody shape could be simply encoded in a gene?
This problem never stopped Burnet thinking that he had the right general principle; he just thought of it as a detail which remained to be understood. Eventually the problem was solved in a series of experiments beginning in the mid-1970s by Japanese-born Susumu Tonegawa, who won the Nobel Prize for this in 1987. The details deserve a whole other book, but in essence Tonegawa discovered (of course building on the work of many others) that antibody genes come in bits that join together in myriad ways. Normally, our genes never get altered in this way. But as each antibody-secreting cell develops in our bone marrow they rearrange these gene segments so that each cell ends up being able to make one antibody – one that’s slightly different from those made by other cells.
But there was a second huge problem with Burnet’s conception of the immune system. Burnet’s Nobel-Prize-winning theory of acquired tolerance – as vindicated by Medawar’s experiments – stated that the immune system learned to not react against our own cells and tissues. Again, what is extraordinary about Burnet’s theory is that he was able to hit upon the right principle even though he could not have initially understood how the process took place in the body.
Insight into how acquired tolerance really works was achieved in 1961 by Jacques Miller, while he was studying for his PhD at the Chester Beatty Research Institute, London. Miller – along with several of his contemporaries – discovered the importance of the thymus, an organ above the heart previously thought to be entirely uninteresting. Containing many dead immune cells, the thymus had simply been written off as the place where cells went to die – a graveyard for immune cells. Miller, however, had a hunch that the thymus was rather more significant.
Far more important than what he set out to study – a particular leukaemia-causing virus – Miller observed that mice with their thymus removed very early in life were unable to fight off all kinds of infections. He immediately grasped the significance of this and then, in skin-graft experiments similar to those done by Medawar, Miller found that mice lacking their thymus could not reject genetically different skin. The thymus, Miller established, was not useless at all – it was crucial for establishing the mouse immune system, and, without it, mice couldn’t fight infections or reject transplanted skin.
But not everyone was convinced. Even after reading Miller’s publications, Medawar himself wrote that ‘we shall come to regard the presence of lymphocytes [i.e. immune cells] in the thymus as an evolutionary accident of no very great significance’.54 Medawar and many other scientific leaders of the time thought, quite simply: how on earth could an organ containing large numbers of dead cells be so vitally important in the creation of the immune system?
Burnet was an exception – he quickly grasped the importance of Miller’s results. In a lecture he gave in London in June 1962, soon after Miller had published his research, Burnet suggested that there are so many dead cells in the thymus because they are all the immune cells that were killed off deliberately because they can be activated by ‘self’ molecules. In this way, Burnet suggested, the thymus is crucial to how the immune system discriminates between self and non-self: only immune cells that don’t react to the body’s own cells and tissues are let out of the thymus alive – and anything recognized by an immune cell that has made it out of the thymus must be something that has never been in your body before.
Pulling all these findings together, we begin to see how Burnet’s theories really work in the body: first, immune cells shuffle segments of genes to define what each immune cell can react against. The outcome of this is that each individual immune cell reacts to a particular shape of molecule, say one found on a germ. However, this reactivity develops randomly, so initially all kinds of immune cells are produced, and some could react to your own cells or tissues. But before an immune cell is let out to patrol the body, the thymus checks that it won’t react to the body’s own cells and tissues. Any that can react to ‘self’ are killed off in the thymus and the rest are let out to defend. An immune cell, now out of the thymus and patrolling the body, seeing a particular germ, is activated to multiply and mounts our defence.
There are, in fact, two different types of immune cells in humans and many animals that rearrange their genes in this special way. Max Cooper at the University of Alabama was one scientist who helped establish this by showing that excision of different organs in birds led to losses in different types of immune cells.55 These different types of immune cells became known as B cells and T cells. B cells are the antibody-secreting cells, named for their development in humans in bone marrow, while T cells are named for being the cells that pass through the thymus in their development.
Without doubt, the research by Burnet, Medawar and their contemporaries was a scientific revolution – one that had been over eighty years in the making, since Pasteur and Koch first established the existence of germs,56 but which moved rapidly in its final stages during the 1950s and early ’60s. From this revolution, major principles for how our body fights disease were established in an explosive synergy between experiments and ideas across three continents: Europe, the US and Australia. Today, acquired tolerance and clonal selection remain central to the understanding of the immune system. They are beautiful aspects of our inner anatomy; hidden from view but as elemental to our well-being as the circulation of blood.
Surprisingly, despite their partnership in establishing this scientific revolution, Burnet and Medawar met only rarely in person. International travel in those decades was relatively uncommon, and Burnet was fifty before he travelled overseas to a scientific conference.57 Their interactions came largely through their formal scientific publications. Even here, their exchange of ideas did not develop systematically. It is tempting – but incorrect – to imagine that Burnet published a hypothesis that Medawar followed up with experiments: the idealized, so-called hypothetico-deductive model of how science works. Medawar himself was very critical of this view of science.58 How any idea develops between people is complicated, and paradigm-changing science is no exception. New knowledge is never attained in a coherent linear sequence of events: only a twisted path leads to something you couldn’t see from the start.
This revolution came from a number of people thinking about the immune system in different ways – each with their own motives and perspective – and the fundamentals of immunology were established only when their contributions were put together. And these basic rules of our immune system were revealed long before much was known about the underlying molecular details. As we’ve seen, Burnet and Jerne developed a number of general principles that proved to be right even though there was very little knowledge available about the actual cells and molecules involved in the human immune system. Similarly, Medawar’s experiments were at the level of physiology rather than in the minutiae of how the immune system worked. This is the opposite way round to how most biological research works now.
Today, we have the tools to be able to manipulate genes and proteins in cells and animals. It’s more common, therefore, to first identify the genetic or molecular requirements of a cell’s behaviour, with general principles being built up afterwards. For example, the way in which cells divide, the subject of a Nobel Prize won by Leland Hartwell, Tim Hunt and Paul Nurse in 2001, was worked out through identification of the molecules and proteins that control the process. This in turn led to the idea that cells move through stages and checkpoints as they divide in two – the cell cycle – and deregulation of this sequence of events can lead to tumour formation. But the likes of Burnet and Medawar worked in a different era of biological science, before genes and proteins could be easily manipulated, which surely adds to the magnitude of their achievements. Theirs was a glorious age when big concepts opened up about the immune system, and the details were left for others to fathom.