Definition of Antigens

A molecule that resides on the surface of a cell membrane and is capable of stimulating an immune response. Antigen molecules are either lipoproteins (lipid and protein) or glycolipids (lipid and glucose). Each cell has numerous antigens that that identify it to the immune system.

Cells that belong to the body bear antigens that mark them as self cells; the immune system does not react to them. The antigens on cells that are foreign to the body alert the immune system to the presence of nonself cells, which activates an immune response. Foreign or nonself antigens cause the immune system to develop antibodies, unique proteins (immunoglobulins) that inactivate or destroy specific antigens.

Blood Type and Antigens

Antigens form the basis of the ABO and rhesus (Rh factor) classification for blood type. Antigens coat the cell membrane surface of erythrocytes (red blood cells) for blood types A, B, and AB. The erythrocytes of type O blood do not have antigens. Erythrocytes may also have Rh antigens, designated as “positive” when used to identify blood type. For example, A+ erythrocytes bear type A and Rh antigens. O- erythrocytes have neither ABO antigens nor Rh antigens.

Antigen Processing

Macrophages, tissue-bound phagocytic white blood cells that start life as monocytes circulating in the blood, are abundant in the lymph tissues. They are the immune cells that sound the alarm to the rest of the immune system that nonself antigens are present. When a macrophage encounters a foreign entity, it surrounds and ingests it. As the macrophage consumes the invader, it displays the invader’s antigens on the surface of its cell membrane.

This display announces the presence of the antigens to other immune cells, notably T-cell lymphocytes, which then mount a full immune response. Other cells that may serve as antigen-presenting cells include B-cell lymphocytes and dendritic cells. Once T-cell lymphocytes “read” the antigen message the macrophage displays, they respond by attacking and killing the invader. Correspondingly, B-cell lymphocytes (plasma cells) generate antibodies to further target the antigen.

Antigens and Cancer

The antigens on the surface of cancer cells have become a focus of much research into early diagnosis and new treatments for cancer. Cancer cells begin as normal cells in the body, bearing self-cell antigens. As the cells change and become cancerous they develop additional antigens. Blood tests can detect some of these antigens, such as prostate specific antigen (psa) on prostate cancer cells. Cancer researchers believe the immune response fails to recognize the mixed antigen population on cancer cells as nonself, which allows the cancer to grow.

For further discussion of antigens within the context of the structures and functions of the immune system, please see the overview section “The Immune System and Allergies.”


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