ABSTRACT
Pathogenic microorganisms have of necessity evolved a wide variety of mechanisms to circumvent host-immune defenses. For example, extracellular pathogens must evade phagocytic cells to survive, and have, therefore, developed a selection of evasive tactics. Antigenic variation is yet another successful protective adaptation for evading host antibodies. Several specialized pathogens, including Borrelia recurrentis, and T. brucei, are able to undergo a series of such antigenic changes during the course of a single infection; Borellia infections are characterized by recurrent episodes, each caused by a newly emergent antigenic variant against which new antibodies must be produced. The IgA proteases are extracellular endopeptidases produced by several species of human pathogens. The IgA proteases exhibit unique substrate specificity for human serum and secretory IgA. Proteolytic enzymes in Gram-positive bacteria often are found to be extracellular.60 However, in Gram-negative bacteria, such enzymes are commonly localized to the periplasmic space.
