About 50 million Americans suffer from allergies, or an overzealous immune system. Instead of ignoring harmless particles, such as pollen and animal dander, the body of an allergic person flags them as intruders and releases inflammatory chemicals to combat them.

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There are four categories of allergies. Type I, or anaphylactic, reactions are the most common and include hay fever, food allergies, and reactions to insect bites. Almost immediately after exposure to the allergen, the body releases a cascade of histamines and leukotrienes. Consequently, airways and nasal linings swell, resulting in congestion, wheezing, and foggy thinking. Treatments include antihistamines and desensitizing shots, in which increasing amounts of the antigen are injected over time until they no longer set off an allergic response. A severe form of type I allergy, called anaphylaxis, is life threatening, because air passageways close and blood vessels rapidly dilate. It’s treated with the hormone epinephrine, which causes the airways to open and the arteries to constrict.

Type II, or cytotoxic, allergic reactions occur on a cellular level; the body is fighting off antigens attached to the surface of cells, such as when rejecting a blood transfusion. A type III, or immune-complex-related, reaction occurs when an antibody- antigen pair becomes deposited in the walls of small blood vessels and results in inflammation and cellular and vascular damage. The most common example of this is rheumatoid arthritis. Last but not least is type IV, or a cell-mediated response, which takes up to 2 days to manifest. This reaction is brought on by an excess of T cells and includes skin allergies and the rejection of transplanted organs.

ADDITIONAL FACTS

  1. The Austrian physician Clemens, Baron von Pirquet (1874–1929), coined the term allergy in the early 1900s from the Greek word allos, which means “other.”
  2. Although many people suspect that they suffer from food allergies, only 2 percent of Americans actually do.
  3. Experts believe that one result of global warming will be an increase in ragweed growth, which will lead to an upsurge in seasonal allergies.