With this background in mind, you are now able to understand what goes on when you become initially sensitized (that is, allergic) to a specific allergen and what is involved in triggering subsequent allergy-attacks at each individual encounter with your allergenic nemesis. To appreciate sensitization the process of developing an allergy let's follow ordinary ragweed pollen grains, the bane of hay fever sufferers, after they enter the respiratory tract.

Once inside, the tiny grains quickly make contact with plasma cells located in the tissues and tiny blood vessels. In response, these cells immediately begin producing IgE antibodies capable of targeting the ragweed pollens. Once produced, the IgE molecules in turn bind to the thousands of surrounding mast cells and basophils to which they have a natural affinity. Somewhere between one hundred to three hundred thousand ragweed-specific IgE molecules may coat each mast cell or basophil. Once this binding process is complete, whether it occurs after the first exposure or the thousandth, the individual is thereafter sensitized to ragweed or, if you prefer, allergic to it.

Now let's examine what happens when a previously sensitized individual (that is, one who is already allergic) is exposed once more to the substance to which he has become allergic. We'll use ragweed as an example again. Once they enter the respiratory system of a hay fever sufferer, the pollens immediately bind to the many IgE molecules on the surfaces of the mast cells and basophils, activating them and triggering the release of many types of chemical mediators, including histamine. When this occurs, symptoms begin.

But that's not the end of the story. Several hours later, in what is called the late-phase reaction, eosinophils and additional basophils become attracted to the allergy site, and these latecomers contribute not only to the severity of the symptoms but to the persistence of the attack. The result of all these processes in our example is the sneezing, runny nose, congestion, and watery eyes of a typical immediate hypersensitivity hay fever attack.

Although constituting by far the most common type of antibody-related allergic reactions, IgE (Type I) allergies are not the only kinds of antibodies responsible for allergy attacks. IgM and IgG antibodies, the body's two major infection-fighting antibodies, are involved in what are called Type II allergy reactions, which involve a slight variation in the steps just described. In these instances, allergens bind first to a specific target tissue (such as red blood corpuscles or platelets), and it is the allergen-cell complex that then attracts the antibodies and results in cellular destruction. Overall, these varieties of allergic reaction, also called cytotoxic reactions because they lead to damage and destruction of cells, are quite rare. When they do occur, they are often linked to the use of sulfa drugs and quinidine.

Type III reactions are the third kind of antibody-mediated allergic responses. Also known as serum sickness disease, these reactions, which also involve IgM and IgG antibodies, differ from the Type II responses in that the antibodies and the allergens bind directly with each other in the bloodstream to form floating allergen-antibody complexes. Eventually these relatively large circulating clumps become trapped in the networks of tiny blood vessels found in the kidneys, lungs, joints, and skin, where they are capable of triggering considerable inflammation and tissue damage.

Finally, in the case of delayed allergic reactions (Type IV), the process of sensitization itself is somewhat different from that described earlier. In Type IV reactions, antibodies are not involved. Rather, direct contact with an allergen, such as poison ivy, induces the development of the allergy by bringing about a permanent alteration in the cell surfaces of T lymphocytes. Once this alteration has taken place, each subsequent reexposure to the offending allergen provokes the sensitized T cells to release the chemical mediators responsible for the itchy, blistering skin rash that results.

How Do Allergies Differ From Normal Immune Responses?
Technically, an allergy is an abnormal immune response. It differs from normal immune reactions in two main ways. First, it is triggered by basically harmless substances, such as pollens, rather than by germs or other threatening organisms or cancers. Second, it tends to be prolonged and out of proportion to what is actually needed to dispose of the offending substance. By contrast, normal immune responses generally match the problem at hand and last no longer than are needed to do the job.

It's Not In Your Head
One point should be clear from the foregoing: Allergies are true physical disorders arising from complex chemical and physical interactions between allergens, specialized cells, antibodies, and chemical mediators. And while allergies are often responsible for a good deal of emotional suffering, they are not, contrary to a popular misconception, emotional diseases. No matter what you have heard, they are not "just in your head."

All the same, heightened nervous tension may play a role in certain allergies. The anxiety, fear, and emotional stress that allergies commonly engender in sufferers, for example, may contribute to the triggering of attacks or the aggravating of existing allergy symptoms, leading to a vicious cycle of suffering. For the many allergy victims who are known to have more than one allergy, the overall picture can be even worse.

Common Reactions Confused with Allergies
True allergies must be distinguished from two other common medical problems with which they are often confused: side effects and intolerance. A side effect is a predictable reaction to a certain medication or food. Antihistamines serve as a good example. The groggi-ness they cause when taken is an expected side effect and is not an allergic reaction. In the same way, the flatulence experienced by many people after consuming beans is a side effect, not an allergic reaction.

An intolerance, on the other hand, is an exaggeration of an expected reaction to a particular food or medication. Continuing with the examples above, if you slept for sixteen hours after taking one antihistamine tablet or if you experienced excessive gas, cramping, or diarrhea after consuming a normal-sized portion of beans, it would be correct to say that you are demonstrating an intolerance to these substances, not an allergy.

The difference between side effects, intolerances, and true allergies is more than academic. Let's say you are aware that you cannot tolerate penicillin, that it gives you stomach cramps. Regardless of that reaction, if you needed penicillin to cure you of some serious infection, you could still safely take it when no satisfactory substitute is available (and suffer the expected cramps). On the other hand, if you developed allergic, life-threatening respiratory problems from taking penicillin, you would not be able to take it. Clearly, then, recognizing the difference between an allergy and a side effect or intolerance can be extremely important and even life-saving.