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.

Image courtesy NIAID

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

Image courtesy U.S. Food & Drug Administration
Common sites for allergic reactions

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.

Anaphylactic Shock
The exposure-reaction time can vary depending on your body (how well you handle the exposure) and the allergen you were exposed to. In a mild case, you may only have mild itching or swelling. In a severe reaction, after exposure to the triggering antigen, you may suddenly develop hives over large areas of your body and begin having breathing difficulties (this is accompanied by a rapid and severe drop in blood pressure). Also, in a severe reaction, thinking becomes muddled as the brain and other vital organs become oxygen-starved. The brain and kidneys are especially vulnerable in this type of reaction and may be permanently damaged even if the victim survives.

To make matters worse, cell fluids dumped into the tissues of the throat can cause the throat to swell shut, leading to anaphylactic shock and death in as little as three or four minutes after exposure to the antigen or the onset of symptoms. Hundreds of people die annually from anaphylactic shock in the United States alone.

Currently, the only effective treatment for anaphylaxis is an intramuscular injection of epinephrine, a hormone the body produces naturally in the adrenal glands. Epinephrine counteracts the symptoms of anaphylaxis by constricting the blood vessels and opening the airways. The down side is that its effects last only 10 to 20 minutes per injection, it has some potentially serious side effects, and it must be administered correctly at or before the onset of symptoms to be effective.