"Innate" or "nonspecific" immunity refers to various physical and cellular attributes that collectively represent the first lines of defense against infectious disease. Obviously, the skin and the extensive epithelial lining of the mucosal tissues act as nonspecific barriers impeding, if not preventing, infectious agents from entering the body. However, once these barriers are penetrated, in a puncture wound for example, infectious agents usually encounter the activities of "defensive" cells, either resident to the site or recruited there from the blood by chemotactic signals which promote diapedesis, or extravasation (Figure 1-5 (in cache). Most notable among the early defensive cells are phagocytes -- mainly macrophages, neutrophils, and dendritic cells -- which destroy an invading organism by phagocytosis, a multi-step process beginning with the complete engulfment of the organism and ending with modification and chemical breakdown of its structural components (Figure 1-4 (in cache)). A host of toxic chemical reactions may occur in phagocytes including biomolecular breakdown by digestive enzymes and chemical modification by highly reactive oxygen (and nitrogen) intermediates generated by the "oxidative burst" reaction.
One consequence of phagocytosis is that macrophages and dendritic cells "present" antigens on their cell surfaces corresponding to the chemical breakdown products of the organism. Antigens presented in this way activate specific immune lymphocytes against the organism and its associated components (Figure 1-14 (in cache). This in turn leads to acquired immunity as defined by the antigen-specific activities of immune lymphocytes and their effects on other cells (Figure 1-8 (in cache) ).
|© 1997, Duane W. Sears|