Antibody
Self reactive antibodies are not found in significant concentration in normal serum because

B cells that are stimulated via their surface bound antibody in the absence of T cell help to commit suicide
it is impossible to generate a self-reactive antibody
self reactive B cells switch to IgA which is all secreted and not present in serum
self reactive B cells are killed by CD8 T cells

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Antibody
The secretary component of sIgA is

made by plasma cells that secrete dimeric IgA
added to dimeric IgA by the M cell
made by T cells
formed by cleavage of an epithelial cell receptor used to transport the dimeric IgA across the epithelial cell

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