An overview of the immune system

By Alan Koenigsberg, M.D.

Several months ago, I wrote a few articles about autoimmune diseases. They included type 1 diabetes, systemic lupus erythematosus, multiple sclerosis and others. Given all of the recent discussions about COVID-19, vaccinations, people getting the flu, colds and the like, I thought a brief overview of how the immune system actually works might be helpful.

The human body has several protective mechanisms in place that help us ward off infections from bacteria, viruses, fungi and parasites. These mechanisms work reasonably well most of the time and we either are able to avoid getting infected in the first place or the protective mechanisms eventually ward off the infections and we heal.

In medicine, we tend to separate the two basic mechanisms of defense into innate and acquired.

Innate mechanisms include the ready-made physical defenses that are present all the time. These include the many layers of the skin, which is the largest organ of the body and literally protects almost the entire person from external invasion most of the time.

There are antibacterial enzymes in saliva in the mouth and acid in the stomach, which also can destroy many invading germs.

The hairs in the nose and cilia in the bronchi and lungs are another example of protections to trap and destroy germs.

There are also innate cell mechanisms that trap and destroy invading germs. Histamine from mast cells in the blood is released locally, when the skin is punctured and inflammation takes place to surround the invading organisms.

This inflammation is well known to cause swelling, pain, warmth and redness, all signs of histamine release. This is the same histamine that causes runny noses, for which we can take antihistamines to prevent excessive inflammation response.

The other main type of immune response is the acquired or adaptive immune response, which comes from cells produced in the bone marrow. This is the type of response we tend to think of when we think of the immune system.

This part of the immune system travels through the blood and is contained within the white blood cells. The red blood cells transport oxygen from the lungs to all the cells in the body and the white blood cells (which can form pus when seen in a skin infection or pimple) are the ones which provide the typical immune response.

The B cells come from the bones, hence “B” cells. They are stem cells from the bone marrow which migrate into the lymph nodes and can produce specific antibodies to invading germs. There are the more rapid B cell response and a memory B cell response, which can be activated very quickly if the same germ invades at a later date.

Antibodies are specific to an invading organism, as differentiated from the T cells, described below.

The T cells — which are lymphocytes that migrate from the bone marrow to the thymus gland in the chest, hence “T” cells — are the ones that provide cellular immunity to invading organisms. T cells can attack and destroy many kinds of bacteria and viruses; they are not as specific as B cells and do not produce antibodies.

The T cells are stored in the secondary lymph glands, such as lymph nodes, spleen, tonsils, appendix and patches in the small intestine.

As far as vaccines: The virus that causes polio, for example, does not seem to mutate and so vaccines may last a lifetime.

Colds, by contrast, may be caused by one of several viruses and they tend to mutate, so a virus to prevent the common cold would not be that useful.

The immune system is a truly amazing and incredibly complex system, which is a specialty field of medicine unto itself.

Alan Koenigsberg, M.D., is a practicing psychiatrist and clinical professor of psychiatry at UTSW Medical School in Dallas. He can be reached at akoenigsberg@mac.com.

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