Notes 4.24b

IgM continued…

*T-cell-independent antibody response = B1, marginal zone B-cells spontaneously producing nonspecific antibody

*T-cell-dependent: antibody production induced by T-cell receptor presenting antigen to B-cell in secondary lymphoid tissue
-reliant on clonal expansion of T-cells

*Like in T-cell receptors, IgM-antigen binding site diversity a result of VDJ recombination
-T-cell presents antigen to B-cell receptor
prolonged contactsomatic hypermutation

Somatic hypermutation:
*cytidine deaminases convert CU
*act on V segment of gene that is coding for this particular B-cell receptor
**V = variability
*VERY high rate of random mutation in V segment
*tweaked B-cell receptor then expresses on B-cell

*Somatic hypermutation of B-cell receptormore perfect match
-This results in “perfect fit” antibodies being secreted by B-cells
(This is another example of positive and negative selection)
*Since IgM secreted first, it binds less well to antigen
*Then, heavy chain gene expression changes-this = isotype switching
-mainly IgG secreted

*Plasma cell = terminally-differentiated B-cell secreting lots of antibody
-can live for several years
-cell maintenance, differentiation because of IL-6
**some perfect fit B-cells don’t become plasma cells, don’t secrete antibody=
memory B-cells, which can live for decades
-upon re-exposure to matching antigen, memory B-cells clonally expand,
differentiate into plasma cells

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