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Disorders of the Immune System

Your immune system is an elaborate system of proteins, cells, organs, and ducts that protects your body from infection by invading microorganisms such as bacteria, viruses, or fungi. When one of these microorganisms enters your body, white blood cells recognize it as foreign and move in to get rid of it. During this process, the white blood cells form a memory of a specific protein—called an antigen—that sits on the surface of the invading microorganism. This memory allows your immune system to recognize the microorganism in the future if you are exposed to it again. Doctors call this memory-forming ability immunity.

Protective immunity keeps you from developing some infections, such as measles, more than once. Your body recognizes the microorganism when it invades again and responds quickly to neutralize it. Immunity also explains how vaccines protect against infectious diseases. Exposure to a small amount of the inactivated microorganism contained in the vaccine creates an immune system memory that your body can use to defend itself if it encounters the microorganism again.

Your immune system fights infection in two important ways, using two types of white blood cells: B cells and T cells. B cells produce antibodies, which are proteins that circulate in the bloodstream. When an invading microbe enters the body, the antibodies target a specific antigen on the surface of the microbe. The antibodies bind to the antigen and either destroy it or change it to make it more attractive to the scavenger white blood cells that will eat it.

T cells play two roles in immune defense. Helper T cells, also called CD4-positive T cells, signal B cells to begin making antibodies. Helper T cells can also activate scavenger white blood cells to gobble up invading microorganisms. Killer T cells, also known as CD8-positive T cells, attack and destroy cells that have been infected by invading microbes.

A number of factors can contribute to the suppression of the immune system and make you susceptible to infection and disease. Stress and malnutrition, including excessive dieting, can adversely affect your body’s ability to launch a defense against infection. Having major surgery or undergoing cancer treatment can also impair the immune system’s ability to function. People who are infected with HIV experience a slow and devastating assault on their immune system that leaves them highly vulnerable to infections and diseases that healthy people can easily fend off.

The Lymphatic System

The organs of your immune system—strategically positioned throughout the body—are called lymphoid organs because they serve as stations for the growth and deployment of lymphocytes (white blood cells that mount the body’s primary defense against infection). Lymphocytes recognize infectious agents and other potentially harmful substances and participate in the body’s immune reaction against them. White blood cells form in the spongy tissue of the bone marrow. This nutrient-rich tissue is found in the center of the long, flat bones, such as those in the pelvis.

The white blood cells travel by way of the blood vessels to the lymphatic system—a circulatory system that is separate from the cardiovascular system. The lymphatic system circulates lymph, a clear liquid that carries white blood cells throughout the body. Round nodules called lymph nodes, or lymph glands, which lie along the lymphatic vessels, supply white blood cells to the bloodstream and remove bacteria and other potentially harmful particles from the lymph. When fighting an infection, the lymph nodes often become swollen. (You have probably noticed swelling of the lymph glands in your neck when you have an infection.)

A number of other key organs are also part of your immune system. The spleen, a fist-sized organ in the upper left part of the abdomen, produces infection-fighting antibodies and some types of white blood cells. Lying deep inside the upper chest behind the breastbone, the thymus changes immature