Antibody
The secretary component of sIgA is

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

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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
self reactive B cells switch to IgA which is all secreted and not present in serum
it is impossible to generate a self-reactive antibody
self reactive B cells are killed by CD8 T cells

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