Immune System and Disorders

The immune system protects the body from invasion by nonself organisms with an array of tissues, cells and organs working together through primary and secondary mechanisms. The primary line of defense is physical and chemical barriers to keep out invaders. The skin forms a tough physical barrier and secretes oil and sweat as chemical defenses. Other chemical defenses are the acid secretions in the stomach and vagina, mucous secretions in the respiratory tract and lysozymes (antibacterial enzymes) in tears. Colonies of friendly bacteria live in the intestines to use up nutrients invaders could thrive on. The flushing action of urine removes invaders from the urinary tract.

If an invader makes it past the first line of defense, a two-pronged secondary attack with nonspecific and specific responses is triggered into action. The nonspecific response is direct and immediate with chemical and cellular agents. The specific response is complex, specific to the invader and takes longer to prepare. The immune system recognizes the body's cells because they carry specific markers for recognition under normal conditions as cells that belong. When the immune system agents encounter an unrecognized invader it triggers the immune response.

To carry out the function of the immune system, organs throughout the body deploy lymphocytes (specialized white blood cells). The organs are often referred to as lymphoid organs and include the bone marrow, thymus, lymph nodes, spleen, tonsils, adenoids, appendix, lymphatic vessels to carry lymphocytes and Peyer's patches clumps of lymphoid tissue in [Page 879]the small intestine. Lymphocytes (B cells and T cells) and phagocytes, the cells which will become immune cells grow in the bone marrow. B cells mature in the bone marrow and T cells migrate to the thymus where they learn to distinguish self-cells from non-self invaders. Mature lymphocytes either collect in the immune tissue and organs or travel in lymphatic vessels throughout the body.