Since immunity and allergy are but two faces of the immune system, you should not be surprised to learn that both kinds of reactions involve many of the same elements. Lymphocytes, for example, are among the specialized white blood cells called into action in both allergic and ordinary immune reactions. One type, the T lymphocyte, works by surrounding foreign invaders or allergens and secreting chemical mediators capable of both destroying the invader and of recruiting additional white blood cells to the affected area to join the battle.
In response to allergens or other foreign materials, a second type of lymphocyte, the 8 lymphocyte, is also often enlisted. Once stimulated, B lymphocytes undergo internal changes to become a new kind of cell, known as a plasma cell, that possesses the ability to manufacture and secrete antibodies into the blood. Antibodies are special types of proteins that work by binding tightly to invading germs or allergens, inactivating them and speeding their removal from the body.
Three other cells, eosinophils, basophils, and mast cells, also figure prominently in immune reactions and allergy. Eosinophils are specialized white blood cells that, under ordinary circumstances, are important for battling large-sized foreign invaders, particularly parasitic worms. However, for reasons that remain unclear, they are also produced in large numbers in certain allergic disorders, especially hay fever. In fact, they are so often seen under the microscope in the nasal and bronchial mucus secretions of hay fever and asthma sufferers that they are commonly referred to by doctors as "allergy cells."
Basophils are another form of white blood cell. Found both in the bloodstream and near the surface of the mucous membranes lining the eyes and nose, these cells contain many kinds of chemical mediators. Some mediators are stored within the cell, and others are produced in response to allergenic stimulation. They are responsible for many tissue changes; for example, they are capable of dilating small blood vessels, stimulating the tiny nerve endings in the mucous membranes, promoting mucus production, and stimulating other tissues to become involved in the overall inflammatory process.
Mast cells are another extremely important type of allergy cell. Ordinarily located deep in the linings of the nose and eyes, these cells typically accumulate in close proximity to blood vessels and mucus-producing cells. Like basophils, mast cells are producers and storers of a wide variety of very potent chemical mediators. They also share with basophils a special affinity for binding IgE molecules (discussed below) to their outer surfaces, a step crucial in the evolution of many types of allergy attacks.
Of the dozen or so mediators so far discovered, histamine is probably best known. Among its many effects, it is responsible for dilating small blood vessels and provoking the inflammation and swelling of tissues. Histamine is believed to provoke such diverse symptoms as the stuffiness, the mucous discharge, and the constricted passageways characteristic of hay fever and asthma attacks and the itchiness of eczemas. For this reason antihistamines drugs that, as the term implies, block the effects of histamine make up an important part of our arsenal of anti-allergy therapies.
