immune disorders
Acquired Immunity
Lymphocytes
Lymphocytes enable the body to remember antigens and to distinguish self from harmful nonself (including viruses and bacteria). Lymphocytes circulate in the bloodstream and Lymphatic System: Helping Defend Against Infection and move into tissues as needed. The immune system can remember every antigen encountered because after an encounter, some lymphocytes develop into memory cells. These cells live a long time—for years or even decades. When memory cells encounter an antigen for the second time, they recognize it immediately and respond quickly, vigorously, and specifically to that particular antigen. This specific immune response is the reason that people do not contract chickenpox or measles more than once and that vaccination can prevent certain disorders. Lymphocytes may be T cells or B cells. T cells and B cells work together to destroy invaders. T cells develop from stem cells in the bone marrow that have travelled to an organ in the chest called the thymus. There, they learn how to distinguish self from nonself antigens so that they do not attack the body's own tissues. Normally, only the T cells that learn to ignore the body's own antigens (self-antigens) are allowed to mature and leave the thymus. T cells can potentially recognize an almost limitless number of different antigens. Mature T cells are stored in secondary lymphoid organs (lymph nodes, spleen, tonsils, appendix, and Peyer patches in the small intestine). These cells circulate in the bloodstream and the lymphatic system. After they first encounter an infected or abnormal cell, they are activated and search for those particular cells. Usually, to be activated, T cells require the help of another immune cell, which breaks antigens into fragments (called Recognition ) and then presents antigen from the infected or abnormal cell to the T cell. The T cell then multiplies and specializes into different types of T cells. These types include Killer (cytotoxic) T cellsattach to antigens on infected or abnormal (for example, cancerous) cells. Killer T cells then kill these cells by making holes in their cell membrane and injecting enzymes into the cells. Helper T cellshelp other immune cells. Some helper T cells help B cells produce antibodies against foreign antigens. Others help activate killer T cells to kill infected or abnormal cells or help activate macrophages, enabling them to ingest infected or abnormal cells more efficiently. Suppressor (regulatory) T cellsproduce substances that help end the immune response or sometimes prevent certain harmful responses from occurring. When T cells initially encounter an antigen, most of them perform their designated function, but some of them develop into memory cells, which remember the antigen and respond to it more vigorously when they encounter it again. Sometimes T cells—for reasons that are not completely understood—do not distinguish self from nonself. This malfunction can result in an Autoimmune Disorders , in which the body attacks its own tissues. B cells are formed in the bone marrow. B cells have particular sites (receptors) on their surface where antigens can attach. B cells can learn to recognize an almost limitless number of different antigens. The main purpose of B cells is to produce antibodies, which tag an antigen for attack or directly neutralize it. B cells can also present antigen to T cells, which then become activated. The B-cell response to antigens has two stages: Primary immune response:When B cells first encounter an antigen, the antigen attaches to a receptor, stimulating the B cells. Some B cells change into memory cells, which remember that specific antigen, and others change into plasma cells. Helper T cells help B cells in this process. Plasma cells produce antibodies that are specific to the antigen that stimulated their production. After the first encounter with an antigen, production of enough of the specific antibody takes several days. Thus, the primary immune response is slow. Secondary immune response:But thereafter, whenever B cells encounter the antigen again, memory B cells very rapidly recognize the antigen, multiply, change into plasma cells, and produce antibodies. This response is quick and very effective. 