Allospecific B cells are activated only after receiving
help from an allospecific T cells recognizing the alloantigen derived
peptide presented by the B cell. Because this interaction is self-restricted,
only T cells that recognize alloantigens in an indirect
way are able to provide the necessary help and no humoral response
can occur when the alloresponse is dominated by T cells recognizing the
alloantigen through the direct
pathway.
The T-B encounter occurs when a recirculating B cell migrates through the T cell zone of the lymph node. Activation of the B cell occurs only when its surface immunoglobulin (Ig) sees the antigen retained on the surface of a dendritic cell. The antigen is then
internalized by the B cell via its surface Ig, processed and presented on
MHC II. The animation illustrates the principle of the “linked” or “cognate” recognition of antigen by T and B cells in a three-cell DC-T-B interaction. The helper T cell must see the same peptide first on a DC and then on a B cell. The B cell receives costimulatory signals from the Th.
Recognition of foreign tissue by alloantibodies
Throughout the movies on allorecognition, proteins (MHC molecules as well as other polymorphic proteins of which the peptides are presented by the MHC-molecules) that differ between responder and stimulator will be depicted in red, while autologous proteins or proteins that are identical in the stimulator are depicted in blue.
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