Why can some people ate wheat with confidence while others suffer? Or, for that matter, why does one individual feel sick after breathing, touching or swallowing something that doesn’t seem to bother the rest of us? The explanation seems to lie in the intricate workings of the immune system, our bodies’ defense against outside invaders.

Although not yet fully understood, immunity is basically out cells’ ability to recognize and destroy anything perceived to be foreign and harmful; bacteria, viruses, fungi and poisons (collectively called antigens). Understanding immunity helps you to understand both the causes and the treatment of allergy.

Think of immunity as a series of defensive actions by your body. ”Search and destroy” missions are controlled by two special kinds of white blood cells called B-lymphocytes (or B-cells) and T- lymphocytes (or T-cells, which hail from the thymus gland). Together, B-cells and T-cells wage war against all invading antigens.

B-cells set up camp in the lymph nodes and bone marrow, launching large numbers of special antigen fighting proteins called antibodies into circulation at the first sign of invasion. T-cells, meanwhile, pitch in by ”pretreating” the invading antigens, changing them into a form easily neutralized or destroyed by antibodies.

It’s a remarkably sophisticated battle plan. The antibodies (called immune bodies or immunoglobulins by scientists) are of five general types: IgA (shorthand for Immunoglobulin A) and, similarly, IgD, IgE, IgG and IgM. Within each group, molecules link up in such an infinite variety of patterns that a specific antibody is produced for just about every possible antigen in the world.

Luckily for us, that ability to produce an unlimited variety of customized antibodies develops without prior exposure to the antigen, be it a virus, bacteria or something else. In other words, antibodies are able to recognized the enemy, sight unseen.

The immune response rolls into actions as soon as any substance perceived as foreign enters the body, whether it’s inhaled, swallowed, touched or injected. As a system for fighting off infections and disease, the antigen antibody reaction works quite well.

And like any good military system, it has reserve plan. Once the activated B-cells have done their initial duty, some continue to reside as ”memory” cells in the body’s lymph glands. There they stand ready for future encounters should the antigens they fought once again trespass into body territory.

Those very same memory cells, in fact, provide continued immunity after our first exposure to certain disease, such as a childhood bout with the mumps or this season’s strain of the flu. However, a popular saying states that is can go wrong, it will – and the immune system provides no exception.

In many allergic people, lymphocytes mistake a perfectly normal and harmless substance like wheat or dust or pollen for an enemy antigen. Inappropriately, the lymphocytes jump into action to initiate plasma production of antibodies – usually the IgE type – against what they seem to regard as the enemy.

Antibodies, in turn, latch on to either of two type of cells within the body: basophils, a type of while blood cell; or mast cells, found in the respiratory tract (nose, throat and lungs), gastrointestinal tract (stomach and intestines) and skin. The IgE loaded cells grow sensitive to that allergen, as the mistaken antigen (be it wheat, dust or whatever) is known.

Whenever the allergen is encountered , sensitized cells release a flood of natural substances – mainly histamine, plus other histaminelike substances called allergy mediators. Normal amounts of those substances do us no harm, but too much tends to expand blood vessels, causing inflammation and discomfort.

If the cells in the nose, eyes and sinuses are affected by histamine or other allergy mediators, the allergic person sneezes, has a runny nose and itchy eyes – the symptoms known as hay fever to you and allergic rhinitis to your doctor. If the skin is affected, hives (or urticaria) appear. If the digestive tract is affected, you suffer with nausea, vomiting or diarrhea.

In the lungs, an allergic response can lead to tightened and clogged airways – the hallmarks of asthma. An allergic reaction can begin immediately – within seconds or minutes – or be delayed for a day or two. If severe enough, any allergic reaction can land you in the hospital for medical help.

The worst reaction by far is what’s called anaphylactic shock: the throat swells shut and the lungs fill with fluid. It’s like trying to breathe with a plastic bag pulled over your head. Fortunately, anaphylactic shock is relatively rare, especially with good allergy management.

In some people, an allergy may not become obvious until late in life when, after repeated contact with the allergen, a person suddenly develops a sensitivity. What happens is that after the first few uneventful encounters, a certain threshold of resistance is surpassed. You might unexpectedly break out one day after eating tomatoes – having eaten then freely all your life.

How One Allergy Can Lead to Another

You can start out with an allergy to, say, wheat and eventually develop allergies to other, related foods like barley, rye or rice, all members of the cereal family. Apparently, antibodies become less choosy and may begin to react with related allergens once the allergic response is established.

Some immunologists believe that cross reactions that were too weak to be noticed at first later surface as antibodies develop greater affinity for that particular kind of antigen. Furthermore, one allergy may exacerbate another, even if they’re not necessarily related.

A person with both milk and pollen allergies, for example, may react to milk only during pollen season, when his or her system is overloaded. Interrelated allergies (coallergies) underlie the importance of considering all possible allergens.