Components
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 antigens, or disease-causing agents.
Macrophages
White blood cells are the main components of the immune system. Some white blood cells, known as macrophages (Monocytes), 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 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. 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, 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.
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.
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.
Antibodies
Antibodies are Y-shaped proteins called immunoglobulins (Ig) and are made only by B Lymphocytes. The antibody binds to the antigen at the ends of the arms of the Y. The area at the base of the Y determines how the antibody will destroy the antigen. This area is used to categorize antibodies into five main classes: IgM, IgG, IgA, IgD, and IgE. During the humoral immune response, IgM is the first class of antibody made. After several days, other classes appear. Exactly which other Ig classes a B cell makes depends on the kind of interleukins it receives from the T helper cells.
Antibodies can sometimes stop an antigen's disease-causing activities simply by neutralization-that is, by binding the antigen and preventing it from interfering with the cell's normal activities. For example, the toxin made by tetanus bacteria binds to nerve cells and interferes with their control of muscles. Antibodies against tetanus toxin stick to the toxin and cover the part of it that binds to nerve cells, thereby preventing serious disease. All classes of antibodies can neutralize antigens.
Antibodies also help destroy antigens by preparing them for ingestion by macrophages in a process called opsonization. In opsonization, antibodies coat the surface of antigens. Since macrophages have receptors that stick to the base of the antibody's Y structure, antigens coated with antibodies are more likely to stick to the macrophages and be ingested. Opsonization is especially important in helping the body resist bacterial diseases.
Finally, IgM and IgG antibodies can trigger the complement system, a group of proteins that cause cells to disintegrate by cutting holes in the cell membrane. Complement is important in resisting bacteria that are hard to destroy in other ways. For example, some of the bacteria that cause pneumonia have a slimy coating, making it hard for macrophages to ingest and eliminate them. However, if IgM and IgG antibodies bind to the pneumonia bacteria and activate the complement system, it is able to cut holes in the bacteria to destroy them.
Although the IgM and IgG classes of antibodies work best in the circulatory system, IgA can exit the bloodstream and appear in other body fluids. IgA is thus important in preventing infection at mucosal surfaces, such as the intestine and the lung. Since these are the sites where most infectious agents enter, IgA is particularly important in resistance to many diseases. IgA is also found in mother's milk and may help nursing newborns resist disease.
