Immune defense against extracellular bacteria

Defense against extracellular bacteria depends on well-functioning mechanisms of phagocytosis, functional antibodies, complement, and intracellular killing.


 * In particular, opsonization with complement components, lectins and antibodies is required to eliminate bacteria . The final elimination is most often provided by neutrophils.
 * Absorbed bacteria are killed intracellularly by oxidative products of NADPH-oxidase (so-called oxidative flare -up ), or by some oxygen-independent mechanisms (lysosomal proteases, nucleases, lipases, lower pH).
 * Cytokines produced by phagocytes, such as interleukin-1, interleukin-6 , TNF , induce fever , metabolic response and increased acute phase protein synthesis.
 * This is followed by stimulation of antigen-specific components of immunity, stimulation first of T-lymphocytes , then B-lymphocytes and IgM production . With the help of T-ly, there is an isotype rearrangement and the promotion of B-ly proliferation and the production of more affine IgG or IgA.

Encapsulated bacterial strains (e.g., pneumococcus ) directly stimulate B cells by aggregating their BCRs and induce T-independent production of IgM antibodies; these, when bound to bacteria, activate the classical complement activation pathway.

Only gram-negative bacteria (eg Neisseria ) are sensitive to the action of the complement membranolytic complex.

Some bacteria produce toxins. Neutralizing antibodies are crucial protection.

IgG and IgA memory antibodies remain in the body after infection, which have a protective role. Memory T and B cells rapidly activate upon further infection and initiate an anamnestic antibody response ( secondary immune response ).

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 * Immune defense against multicellular parasites
 * Immune defense against intracellular bacteria and fungi