Immune Responses to Extracellular and Intracellular Bacteria Can Differ

Infection by extracellular bacteria induces production of humoral antibodies, which are ordinarily secreted by plasma cells in regional lymph nodes and the submucosa of the respiratory and gastrointestinal tracts. The humoral immune response is the main protective response against extracellular bacteria. The antibodies act in several ways to protect the host from the invading organisms, including removal of the bacteria and inactivation of bacterial toxins (Figure 17-8). Extracellular bacteria can be pathogenic because they induce a localized inflammatory response or because they produce toxins. The toxins, endotoxin or exotoxin, can be cytotoxic but also may cause pathogenesis in other ways. An excellent example of this is the toxin produced by diphtheria, which exerts a toxic effect on the cell by blocking protein synthesis. Endotoxins, such as lipopolysaccharides (LPS), are generally components of bacterial cell walls, while exotoxins, such as diphtheria toxin, are secreted by the bacteria.

Antibody that binds to accessible antigens on the surface of a bacterium can, together with the C3b component of complement, act as an opsonin that increases phagocytosis and thus clearance of the bacterium (see Figure 17-8). In the case of some bacteria—notably, the gram-negative organisms—complement activation can lead directly to lysis of the organism. Antibody-mediated activation of the complement system can also induce localized production of immune effector molecules that help to develop an amplified and more effective inflammatory response. For example, the complement split products C3a, C4a, and C5a act as anaphy-latoxins, inducing local mast-cell degranulation and thus vasodilation and the extravasation of lymphocytes and neu-trophils from the blood into tissue space (see Figure 17-8). Other complement split products serve as chemotactic factors for neutrophils and macrophages, thereby contributing to the buildup of phagocytic cells at the site of infection. Antibody to a bacteria toxin may bind to the toxin and neutralize it; the antibody-toxin complexes are then cleared by phagocytic cells in the same manner as any other antigen-antibody complex.

While innate immunity is not very effective against intra-cellular bacterial pathogens, intracellular bacteria can activate NK cells, which, in turn, provide an early defense against these bacteria. Intracellular bacterial infections tend to induce a cell-mediated immune response, specifically, delayed-type hypersensitivity. In this response, cytokines secreted by CD4+ T cells are important—notably IFN-7, which activates macrophages to kill ingested pathogens more effectively (see Figure 14-15).

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  • Kimberly
    Can extracellular bacteria act as intracellular?
    7 years ago
  • kevin
    Which innate response to extracellular bacteria?
    2 years ago
  • michael
    Which immune system work on extracellular bacterial?
    11 months ago
  • Hugo
    How does immune response to extracellular and intracellular bacteria differ?
    8 months ago
  • Tomi
    How immune rsponce to exrta cellular ognanism differ from immune rsponce to inta cellualr organism?
    6 months ago
  • kaj
    How the adaptive immune response to extracellular bacterial pathogen?
    27 days ago

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