Antibody Reactions, Immune Response, and Immunologic Memory

Antigen-antibody reactions

Ag + Ab AgAb

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·reversible reaction as there are formed noncovalently biochemical bonds

·strength of such interactions is called affinity

·different immunoglobulins within an individual show a wide range of affinity

·valence - number of epitope-binding sites of one immunoglobulin molecule

·avidity - describes the collective affinity of multiple binding sites (affinity + valence)

·e.g. although IgM has a low affinity, its avidity is quite high due to 10 epitope-binding sites

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Primary and secondary immune response

·primary immune response

ofirst contact to a foreign antigen that has to be eliminated from the body

ofirst a lag/induction phase (ca. 7-9 days or even up to months) occurs in which no antibody is produced, but activated B cells are differentiating into plasma cells

oamount of IgM antibody is usually very low

oIgM is present in plasma only (isohaemagglutinins belong to this class)

Secondary immune response

osecond and subsequent exposure to the same antigen again

omore rapid (short induction phase) and vigorous

oIgG antibodies are now produced mostly and in quite high amounts in comparison to IgM in the primary response

·a difference between primary and secondary immune responses is an evidence for immunologic memory

·this phenomenon can only be seen in adaptive immunity

·once an infectious organism stimulates an adaptive response, subsequent encounters with that organism often produce mild or unnoticeable effects because of the rapid and enhanced action of antibodies of effector T cells

·memory cells can rapidly be mobilized in secondary immune response, thus shortening the response time to antigen

·isotype switch of antibodies produced by B-cells induced by type 2 cytokines if exposed repeatedly to the same epitope

·now activation of other mechanisms is possible (e.g. complement activation by IgM & IgG, secretion into external body fluids by IgA, mast cell degranulation by IgE)

·IgM is predominant isotype in primary immune response

·mostly IgG, with IgA and IgE, in secondary immune response

·by repeated exposure to antigen isotype switch leads to changes in binding efficiency of the antibodies (by small mutations in the DNA encoding variable regions of L- and H-chains)

·affinity maturation - B cells bearing mutations that result in tighter binding of epitopes by their surface immunoglobulins proliferate more rapidly, thus continuously dominate all other B cells and produce a higher overall affinity against the epitope

·the mechanism of immunologic memory is used in vaccination

oprimary immune response is triggered by „crippled“ microbes (non-virulent)

omemory cells (B- and T-cells) build a defensive reservoir for future assaults

oin future exposures to this microbe, even in virulent form, the body is already armed and is able to react quickly and with great vigor (secondary immune response)