The adaptable system of body defences centres on specialized white blood cells called lymphocytes. These respond to invasion by varied microorganisms. The complexities of the system aim to create the condition of immunity, in which, after the first attack, the body is protected or resistant to future incursions by each particular type of microorganism.
The lymph nodes (or “glands”) are vital to the body’s defence system – they produce and harbour immune cells (lymphocytes) that protect the body from disease. Lymph nodes are scattered throughout the body and also concentrated in groups (see illustration) . Each node is a mass of lymphatic tissue divided into compartments by partitions of connective tissue known as trabeculae. Lymph fluid from most tissues or organs flows through one or more lymph nodes, where it is filtered and cleaned, before draining into the venous bloodstream. Several smaller lymphatics (vessels) bring lymph to the node, and one larger vessel carries it away. The lymph vessels have valves to ensure a one-way flow of fluid.
The immune responses involve attacks on specific micro-organisms or the toxins (harmful substances) they produce, as shown in the section on specific response. Non-specific responses react to any kind of damage, such as a physical knock, a burn, extreme cold, corrosive chemicals, and various forms of radiation, as well as living invaders, ranging from microbes to large parasites, such as worms and flukes. The main non-specific defensive response is inflammation (see section on Inflammatory Response) . Damaged tissue releases chemicals that attract white blood cells. The walls of the capillaries, the smallest blood vessels at the site, become more permeable and porous to allow the passage of white blood cells, defensive chemicals, and fluids, which accumulate as the battle proceeds. The white blood cells surround, engulf, and destroy the invading pathogens. The blood may also clot to form a barrier that not only seals the leak but also prevents further microbial penetration.
Specific responses may occur alongside non-specific reactions such as inflammation, or follow if the infection persists. There are two main types of specific defence: cell-mediated and antibody-mediated (humoral) immunity. Both depend on the actions of lymphoctyes of two different kinds – B and T lymphocytes. B cells make protein antibodies known as gammaglobulins. These react against antigens (foreign protein substances), which differ from the body’s own natural proteins. T cells multiply and attack the pathogen cells.
The cellular or cell-mediated type of immunity involves various types of T lymphocyte or T cell, named because they develop inside the thymus gland. Once a T cell recognizes the antigen, it multiplies rapidly and its offspring differentiate into several types. Helper T cells activate both B cells to help antibody-mediated immunity and macrophages to engulf the microbes and debris. Killer (cytotoxic) T cells attack microorganisms as well as any body cells infected with them, using powerful proteins called lymphokines. Suppressor T cells inhibit the response of other cells to the invading microbes.
Whereas T cells can attack invading organisms directly, the lymphocytes called B cells do so “remotely”’ by producing chemicals called antibodies. These are generally Y or T shaped. Each type of antibody acts against a certain microorganism or “non-self” material by attaching to antigens on its surface. The presence of antigens triggers B cells to multiply. Some develop into plasma cells, which are the main antibody-producing cells. As with cell-mediated immunity, memory cells are produced, which can recognize the same antigen and initiate defence many years later.
Circulating in the blood are over 20 proteins and related substances or factors that form the complement system. The complement (or helping and enhancing) proteins are activated by antibodies, certain lymphokines made by the lymphoctyes called T cells (as illustrated here), bits of cell membrane or DNA, or other products of the battle against invading microorganisms. Once a complement reaction begins, it continues in a “cascade” fashion with one complement protein activating the next, and so on (similar to the cascade reactions of blood clotting). The complement system generally helps to destroy microbes, and prevent them attacking body cells, encourage the activity of white cells such as macrophages, widen blood vessels, and clear away the antigen–antibody complexes.
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