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How Allergies Work

Allergic Reactions

Image courtesy NIAID

Although mast cells are found in connective tissue and basophils are a type of white blood cell, they have one thing in common to the allergy sufferer. They contain histamine, an important weapon in the body's arsenal for fighting infection. Unfortunately, when released into the body inappropriately or in too high a quantity, histamine is a potentially devastating substance.

The Allergic Cascade

It takes between a week and 10 days of sensitizing exposure for the mast cells and basophils to become primed with IgE antibodies. Then, if the allergen comes along again, it triggers a destructive domino effect within the system called the allergic cascade.


Whether it's a protein molecule on a ragweed pollen particle that has been inhaled, or the injected protein in the venom of a wasp, the same sequence of events takes place:

  • The IgE antibodies bound to the surfaces of basophils and mast cells recognize the protein surface markers of the allergen.
  • The IgE antibodies react by binding to the protein surface markers while remaining attached to the mast cells or basophils.
  • This binding alerts a group of special proteins called the complement complex that circulates in the blood.

There are about 20 proteins in this family of proteins, at least nine of which are involved in the allergic-response mechanism. After the IgE antibody (which is already attached to a mast cell or basophil) encounters and binds to its specific allergen, the first complement protein attaches itself to the site. This alerts the next complement protein in the sequence, which joins and alerts the next, and so on. When the string is complete, the offending cell is destroyed. This is fine in a normal immune system, as Ig antibodies latch onto surface markers of disease cells and cause their destruction. But in an allergic episode, the cells involved are mast cells and basophils.

When mast cells and basophils are destroyed, their stores of histamine and other allergy mediators are released into the surrounding tissues and blood. This causes dilation of surface blood vessels and a subsequent drop in blood pressure. The spaces between surrounding cells fill with fluid. Depending on the allergen or the part of the body involved, this brings on the various allergy symptoms, some of the most common being:

  • Itching (body, eyes, nose)
  • Hives
  • Sneezing
  • Wheezing
  • Nausea
  • Diarrhea
  • Vomiting

Cross Reactivity

Although the exact mechanism isn't yet understood, allergy sufferers sometimes find that once they have become sensitized to certain allergens, they also exhibit allergic symptoms when exposed to related substances. For example, if you have an allergic reaction to honeybee venom, you might also test positive for hypersensitivity to all other types of bee venom.

Systemic Reactions

Some allergic people become sensitized to proteins in such things as ragweed pollen, latex, certain foods, and drugs like penicillin. With these allergies, the reaction can involve the entire body. This is called a systemic reaction, and is what your doctor is watching for when you are asked to wait around for a while after an injection. In a systemic reaction, the release of allergy mediators (chief among them being histamine) causes capillaries all over the body to dilate. If this proceeds to the point of danger, it is known as anaphylaxis. If it proceeds even further, the victim passes into anaphylactic shock. See the next page to learn more.