immune disorders
Overview of the Immune System
Lines of Defense
The body has a series of defenses. Defenses include Physical barriers White blood cells Molecules such as antibodies and complement proteins Lymphoid organs The first line of defense against invaders is mechanical or physical barriers: The skin The cornea of the eyes Membranes lining the respiratory, digestive, urinary, and reproductive tracts As long as these barriers remain unbroken, many invaders cannot enter the body. If a barrier is broken—for example, if extensive burns damage the skin—the risk of infection is increased. In addition, the barriers are defended by secretions containing enzymes that can destroy bacteria. Examples are sweat, tears in the eyes, mucus in the respiratory and digestive tracts, and secretions in the vagina. The next line of defense involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and other invaders. This defense has two parts: Innate immunity Acquired immunity Innate Immunity : Phagocytes ingest invaders. Phagocytes include Monocytes and Macrophages , Natural Killer Cells are formed ready to recognize and kill cancer cells and cells that are infected with certain viruses. Some white blood cells (such as Basophils and Acquired Immunity : Innate immunity and acquired immunity interact, influencing each other directly or through molecules that attract or activate other cells of the immune system—as part of the Activation and mobilization . These molecules include Cytokines (which are the messengers of the immune system) Antibodies None Complement proteins (which form the Complement System ) These substances are not contained in cells but are dissolved in a body fluid, such as plasma (the liquid part of blood). Some of these molecules, including some cytokines, promote inflammation. Inflammationoccurs because these molecules attract immune system cells to the affected tissue. To help get these cells to the tissue, the body sends more blood to the tissue. To carry more blood to the tissue, blood vessels expand and become more porous, allowing more fluids and cells to leave blood vessels and enter the tissue. Inflammation thus tends to cause redness, warmth, and swelling. The purpose of inflammation is to contain the infection so that it does not spread. Then other substances produced by the immune system help the inflammation resolve and damaged tissues heal. Although inflammation may be bothersome, it indicates that the immune system is doing its job. However, excessive or long-term (chronic) inflammation can be harmful. The immune system includes several organs in addition to cells dispersed throughout the body. These organs are classified as primary or secondary lymphoid organs. Theprimary lymphoid organsare the sites where white blood cells are produced and/or multiply: Thebone marrowproduces all the different types of white blood cells, including neutrophils, eosinophils, basophils, monocytes, B cells, and the cells that develop into T cells (T cell precursors). In thethymus,T cells multiply and are trained to recognize foreign antigens and to ignore the body’s own antigens. T cells are critical for acquired immunity. When needed to defend the body, the white blood cells are mobilized, mainly from the bone marrow. They then move into the bloodstream and travel to wherever they are needed. The lymphatic system is a vital part of the immune system, along with the thymus, bone marrow, spleen, tonsils, appendix, and Peyer patches in the small intestine. The lymphatic system is a network of lymph nodes connected by lymphatic vessels. This system transports lymph throughout the body. Lymph is formed from fluid that seeps through the thin walls of capillaries into the body's tissues. This fluid contains oxygen, proteins, and other nutrients that nourish the tissues. Some of this fluid reenters the capillaries and some of it enters the lymphatic vessels (becoming lymph). Small lymphatic vessels connect to larger ones and eventually form the thoracic duct. The thoracic duct is the largest lymphatic vessel. It joins with the subclavian vein and thus returns lymph to the bloodstream. Lymph also transports foreign substances (such as bacteria), cancer cells, and dead or damaged cells that may be present in tissues into the lymphatic vessels and to lymph nodes for disposal. Lymph contains many white blood cells. All substances transported by the lymph pass through at least one lymph node, where foreign substances can be filtered out and destroyed before fluid is returned to the bloodstream. In the lymph nodes, white blood cells can collect, interact with each other and with antigens, and generate immune responses to foreign substances. Lymph nodes contain a mesh of tissue that is tightly packed with B cells, T cells, dendritic cells, and macrophages. Harmful microorganisms are filtered through the mesh, then identified and attacked by B cells and T cells. Lymph nodes are often clustered in areas where the lymphatic vessels branch off, such as the neck, armpits, and groin. Thesecondary lymphoid organsinclude the Spleen Lymph nodes Tonsils Appendix Peyer patches in the small intestine These organs trap microorganisms and other foreign substances and provide a place for mature cells of the immune system to collect, interact with each other and with the foreign substances, and generate a specific immune response. The lymph nodes are strategically placed in the body and are connected by an extensive network of lymphatic vessels—the Overview of the Lymphatic System . The lymphatic system transports microorganisms, other foreign substances, cancer cells, and dead or damaged cells from the tissues to the lymph nodes, where these substances and cells are filtered out and destroyed. Then the filtered lymph is returned to the bloodstream. Lymph nodes are one of the first places that cancer cells can spread. Thus, doctors often evaluate lymph nodes to determine whether a cancer has spread. Cancer cells in a lymph node can cause the node to swell. Lymph nodes can also swell after an infection because acquired immune responses to infections are generated in lymph nodes. Sometimes lymph nodes swell because bacteria that are carried to a lymph node are not killed and cause an infection in the lymph node (lymphadenitis). Lymph nodes contain a mesh of tissue where harmful microorganisms and dead or damaged cells are filtered out and destroyed. 