Infection and Immune System

Different types of infectious agents stimulate distinct patterns of immune responses and have evolved unique mechanisms for evading specific immunity. The principal protective immune response against extracellular bacteria consists of specific antibodies which opsonise the bacteria for phagocytosis and activate the complement system. Toxins produced by such bacteria are also neutralized and eliminated by specific antibodies. Same bacterial toxins are powerful inducers of cytokine production, an cytokine account for much of the systemic pathology associated with severe, disseminated infections with these microbes. Intracellular bacteria are capable of surviving and replicating within host cells, including phagocytes. Immunity againts these microbes is principally cell-mediated and consists of CD4 + cells activating macrophages as well as CD8 + cytolytic T lymphocytes. The characteristic pathologic response to infection by intracellular bacteria is granulamatous inflammation. Viruses are obligatory intracellular microbes. Natural immunity againts viruses is mediated by Type I interferons and NK cells. Specific antibodies protect against viruses early in the course of infection. The major defence mechanisim against established infections consist of specific CTLs. Animal parasites, such as protozoa and helmints, give rise to chronic and persistent infections, because natural immunity against them is weak and because parasites have evolved multiple mechanisms for evading and resisting specific IgE antibodies and eosinophilia, granulomatous inflammation, cytokine production and specific CTLs. The chronicity of parasitic infestations often leads to secondary immunopathologic consequences including the formation of immune complexes and the development of autoimmunity