The Human Body

Components of the Immune System

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The ability of the immune system to mount a response to disease is dependent on many complex interactions between the components of the immune system and the antigens on the invading pathogens, or disease-causing agents.

A. Macrophages
White blood cells are the mainstay of the immune system. Some white blood cells, known as macrophages, play a function in innate immunity by surrounding, ingesting, and destroying invading bacteria and other foreign organisms in a process called phagocytosis (literally, “cell eating”), which is part of the inflammatory reaction. Macrophages also play an important role in adaptive immunity in that they attach to invading antigens and deliver them to be destroyed by other components of the adaptive immune system.

B. Lymphocytes

Lymphocytes are specialized white blood cells whose function is to identify and destroy invading antigens. All lymphocytes begin as “stem cells” in the bone marrow, the soft tissue that fills most bone cavities, but they mature in two different places. Some lymphocytes mature in the bone marrow and are called B lymphocytes. B lymphocytes, or B cells, make antibodies, which circulate through the blood and other body fluids, binding to antigens and helping to destroy them in humoral immune responses.

Other lymphocytes, called T lymphocytes, or T cells, mature in the thymus, a small glandular organ located behind the breastbone. Some T lymphocytes, called cytotoxic (cell-poisoning) or killer T lymphocytes, generate cell-mediated immune responses, directly destroying cells that have specific antigens on their surface that are recognized by the killer T cells. Helper T lymphocytes, a second kind of T lymphocyte, regulate the immune system by controlling the strength and quality of all immune responses.

Most contact between antigens and lymphocytes occurs in the lymphoid organs—the lymph nodes, spleen, and tonsils, as well as specialized areas of the intestine and lungs (see Lymphatic System). Mature lymphocytes constantly travel through the blood to the lymphoid organs and then back to the blood again. This recirculation ensures that the body is continuously monitored for invading substances.

C. Antigen Receptors

One of the characteristics of adaptive immunity is that it is specific: Each response is tailored to a specific type of invading antigen. Each lymphocyte, as it matures, makes an antigen receptor—that is, a specific structure on its surface that can bind with a matching structure on the antigen like a lock and key. Although lymphocytes can make billions of different kinds of antigen receptors, each individual lymphocyte makes only one kind. When an antigen enters the body, it activates only the lymphocytes whose receptors match up with it.

D. Antigen-Presenting Cells

When an antigen enters a body cell, certain transport molecules within the cell attach themselves to the antigen and transport it to the surface of the cell, where they “present” the antigen to T lymphocytes. These transport molecules are made by a group of genes called the major histocompatibility complex (MHC) and are therefore known as MHC molecules. Some MHC molecules, called class I MHC molecules, present antigens to killer T cells; other MHC molecules, called class II MHC molecules, present antigens to helper T cells.