- encountering them in the surrounding lymph or
- by being presented them by macrophages or dendritic cells.
B lymphocytes process antigen by the class II pathway for presentation to T cells.
The process:- B cells engulf antigen by receptor-mediated endocytosis
- The B cell receptors for antigen (BCRs) are antibodies anchored in the plasma membrane.
Link to discussion of how BCRs are synthesized. - The affinity of these for an epitope on an antigen may be so high that the B cell can bind and internalize the antigen when it is present in body fluids in concentrations thousands of times smaller than a macrophage would need.
- The remaining steps of antigen processing occur by the same class II pathway described above for macrophages producing
- fragments of antigen displayed at the cell surface nestled in the groove of class II histocompatibility molecules.
- A CD4+ T cell that recognizes the displayed antigen is stimulated to release lymphokines.
- These, in turn, stimulate the B cell to enter the cell cycle.
- Because of the part they play in stimulating B cells, these CD4+ T cells are called Helper T cells ("Th").
- The B cell grows into a clone of cells (called plasma cells)
- These synthesize receptors (BCRs) with the identical binding site for the epitope but without the transmembrane tail.
- These antibodies are secreted into the surroundings.
