Cell-mediated immunity In cell-mediated immunity, T cells respond directly to antigens (foreign substances, such as bacteria or toxins, that induce anti- body formation). This response involves destruction of target cells—such as virus-infected cells and cancer cells—through the secretion of lymphokines (lymph proteins). Examples of cell- mediated immunity are rejection of transplanted organs and de- layed immune responses that fight disease. Thirty-six percent of white blood cells (WBCs) are T cells. They are thought to originate from stem cells in the bone marrow; the thymus gland controls their maturity. In the process, a large number of antigen-specific cells are produced. A license to kill, help, or suppress T cells can be killer, helper, or suppressor T cells. • Killer cells bind to the surface of the invading cell, disrupt the membrane, and destroy it by altering its internal environment. • Helper cells stimulate B cells to mature into plasma cells, which begin to synthesize and secrete immunoglobulin (pro- teins with known antibody activity). • Suppressor cells reduce the humoral response. Humoral immunity B cells act in a different way than T cells to recognize and destroy antigens. B cells are responsible for humoral or immunoglobulin-mediated immunity. B cells originate in the bone marrow and mature into plasma cells that produce antibodies (immunoglobulin molecules that in- teract with a specific antigen). Antibodies destroy bacte- ria and viruses, thereby preventing them from entering host cells. Get to know your immunoglobulin Five major classes of immunoglobulin exist: Immunoglobulin G (IgG) makes up about 80% of plasma anti- bodies. It appears in all body fluids and is the major antibacterial and antiviral antibody. Immunoglobulin M (IgM) is the first immunoglobulin produced during an immune response. It’s too large to easily cross mem- brane barriers and is usually present only in the vascular system. 341 UNDERSTANDING THE IMMUNE SYSTEM My job is to produce antibodies that, in turn, will attack an antigen. Keep in mind that I attack the antigen directly… …while I produce antibodies that incapacitate the antigen.
Immunoglobulin A (IgA) is found mainly in body secretions, such as saliva, sweat, tears, mucus, bile, and colostrum. It defends against pathogens on body surfaces, especially those that enter the respiratory and GI tracts. Immunoglobulin D (IgD) is present in plasma and is easily bro- ken down. It’s the predominant antibody on the surface of B cells and is mainly an antigen receptor. Immunoglobulin E (IgE) is the antibody involved in immediate hypersensitivity reactions, or allergic reactions that develop with- in minutes of exposure to an antigen. IgE stimulates the release of mast cell granules, which contain histamine and heparin.
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- Fall '15