Nonspecific immune protection is provided by such mechanisms as phagocytosis, fever, and the release of interferons. Specific immunity, which involves the functions of lymphocytes, is directed at specific molecules, or parts of molecules, known as antigens.
The immune system includes all of the structures and processes that provide a defense against potential pathogens (disease-causing agents). These defenses can be grouped in two categories: innate (or nonspecific) immunity and adaptive (or specific) immunity. Although these two categories refer to different defense mechanisms, there are areas in which they overlap.
Innate, or nonspecific, defense mechanisms are inherited as part of the structure of each organism. Epithelial membranes that cover the body surfaces, for example, restrict infection by most pathogens. The strong acidity of gastric juice (pH 1-2) also helps to kill many microorganisms before they can invade the body. These external defenses are backed by internal defenses, such as phagocytosis, which function in both a specific and nonspecific manner (table 15.1).
Each individual can acquire the ability to defend against specific pathogens by prior exposure to those pathogens. This adaptive, or specific, immune response is a function of lymphocytes. Internal specific and nonspecific defense mechanisms function together to combat infection, with lymphocytes interacting in a coordinated effort with phagocytic cells.
The genes required for innate immunity are inherited. Since this limits the number of genes that can be devoted to this task, innate immune mechanisms combat whole categories of pathogens. A category of bacteria (called gram-negative), for example, can be recognized by the presence of particular molecules (called lipopolysaccharide) on their surfaces. In adaptive immunity, by contrast, specific features of pathogens are recognized. The enormous number of different genes required for this task is too large to be inherited. Instead, the variation is produced by genetic changes in lymphocytes during the life of each person after birth.
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