BACTERIAL EVASION OF IMMUNE DEFENSE
There are several ways in which bacteria can
survive in the host by evasion of the immune defenses. These mechanisms will be
briefly summarized here, but the list of evasive mechanisms used by each
bacterial species may be much longer in nature.
Capsules play an important role for long-term
survival of pathogens. For example, group A streptococci have a hyaluronic acid
capsule that is identical to hyaluronic acid in humans. While antibod-ies can
be obtained following immuniza-tion with the streptococcal hyaluronic acid,
they are nonprecipitating antibodies and are not effective in eliminating the
organ-ism. Polysaccharide antigens of both the pneumococcus and the
meningococcus capsules can inhibit phagocytosis of the organism and mucoid
secretions of these polysaccharides can block the activation of the alternate
pathway of complement.
Antigenic variation or drift is another mechanism
whereby bacteria evade the immune system. The M protein (the most important
virulence factor of the group A streptococcus) has been shown to exhibit
antigen variation in the environment, and new M protein molecules appear
regularly in human isolates of group A streptococci.
Another example is the relapsing fever by Borrelia. During the first episode,
anti-bodies kill the bacteria and the fever sub-sides. However, some antigenic
variants of the bacteria persist, and after five to seven days these new
variants can cause a relapse in the patient with fever reap-pearing. Other
examples of evasion of the immune response are bacteria such as N. gonorrhea, N. meningitidis, Haemophilus
Table 4.2 List
of All the Known Superantigens and Their Vβ Specificities
One must also consider that some bac-teria
sequester themselves in nonphago-cytic cells where they are not exposed to the
immune system. An excellent example is M.
tuberculosis, which can lie dormant for years inside a granulomatous, case-ous lesion
called the Ghon complex. Once ground up, live M. tuberculosis organism can be extracted from this complex.
Finally, a number of organisms display antigens on
their surface that are cross-reactive with human antigens. This could result in
an enhanced immune response to host tissue antigens or a diminished response
secondary to similarities between bacterial and self-antigens.
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