Fig. 13: An Effector T4-Lymphocyte Recognizing Epitope/MHC-II on an activated B-Lymphocyte and Producing Cytokines to Trigger Proliferation and Differentiation

Exogenous antigens are those from outside cells of the body. Examples include bacteria, free viruses, yeasts, protozoa, and toxins. These exogenous antigens bind to B-cell receptors and enter B-lymphocytes through endocytosis. After lysosomes fuse with the phagosome, protein antigens are degraded by proteases into a series of peptides. These peptides eventually bind to grooves in MHC-II milecules and are transported to the surface of the B-lymphocyte. Effector T4-lymphocytes are then able to recognize the peptide/MHC-II complexes by means of their T-cell receptors (TCRs) and CD4 molecules.

This enables the effector T4-lymphocytes to produce cytokines such as interleukin-2 (IL-2) , interleukin-4 (IL-4), interleukin-5 (IL-5), and interleukin-6 (IL-6). Collectively these cytokines:

1) Enable activated B-lymphocytes to proliferate;
2) Stimulate activated B-lymphocytes to synthesize and secrete antibodies;
3) Promote the differentiation of B-lymphocytes into antibody-secreting plasma cells; and
4) Enable antibody producing cells to switch the class or isotypeof antibodies being produced.

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