The Immune System

An allergy is actually a category of hypersensitivity. Substances (called allergens) that most people would suffer no problem from cause others' immune systems to go into overdrive. Here's what's happening 'under the hood'...

There is a system in the body that runs parallel to the blood vessels, called the lymph system. It plays a major role in keeping disease and infections at bay. Much of that is concentrated in the lymph nodes, part of the system that produces lymphocytes, cells that attack disease.

Tonsils, the appendix and others are also lymphoid tissue. But there is so-called lymphoid tissue elsewhere, including most importantly the bone marrow and the thymus (an organ just behind the breastbone). It is the thymus that produces T cells, a type of white blood cell. These T cells play a key role in allergies.

As a result of many articles written on AIDS over the years, the name of these cells is widely recognized. But what do they do?

They have several functions, but one of the primary ones is to patrol the blood and lymphatic fluid looking for foreign substances. Every cell in our bodies has a genetic 'signature' that is unique to us. Anything, like a bacteria or virus, that has a different signature is seen as a foreign substance and a candidate for removal.

Some T cells merely mark those foreigners as foreign, then the substances are removed by phagocytes or other mechanisms. Some T cells communicate with B cells to help them do their job in the immune system. And some T cells directly attack those foreign proteins.

Mast cells (a type of white blood cell), for example, in conjunction with basophils (another type) rush to the site of such an invader. On cue, they cause biochemicals such as histamines and prostaglandins to flood the area. That helps engulf the invader, neutralizing it. The 'package' is then carried away and out of the body.

But that reaction can go too far. If an otherwise harmless substance gets marked for destruction, the whole process is carried out unnecessarily. If the reaction is larger than needed, too much histamine and other substances can be released.

In either case, the result is an allergic reaction.

The runny nose from a nasal inflammation (allergic rhinitis), red and watery eyes from inflamed eyelid membranes (conjunctivitis), red skin welts (hives) or other common allergy symptoms all follow from those processes. The body is overreacting, causing the immune system to go into hyperdrive and cause harm to itself.

Building up the immune system is one major key to good health. But like anything, balance is essential. Too weak a reaction would leave us vulnerable to disease. Too strong a reaction creates an allergy.