chapter 35
Molecular Immunology
(Also see color figure.) Antigen recognition by B-cell surface receptors. Response by B cells begins with antigen
binding to receptors on the B-cell surface (membrane-bound IgM and/or IgD molecules). Phagocytosis by the B cell
transports the antigen into the cell where it is degraded to peptides by proteolytic enzymes. The antigen-derived peptides
are next transported to the surface and bind to membrane proteins of the MHC. The peptides are presented by the MHC
proteins (specifically MHC II) to T-helper (Th)cells. The Th cells bind to MHC II proteins via a specific T-cell receptor
(TcR) and the peptides displayed on MHC II molecules. Interleukin-4 from the Th cells (marked by the surface protein
CD4) stimulate the B cells to proliferate and to synthesize and secrete antibodies.
(Also see color figure.) Antigen-antibody complex formation and
recognition by macrophages. Antigens that react directly with antibodies in
the circulation and in the extravascular fluid are recognized and removed by
macrophages. Recognition occurs with the Fc region of the antibody in the
Ag-Ab complex. The antigen-antibody complex is bound to Fc receptors
on the external surface of the macrophage. The antigen is taken into the
cell by endocytosis and degraded proteolytically. The antigen-derived
peptides are transported and bound to the surface MHC II proteins. The
MHC II peptide complexes are presented to the antigen-specific T cells.
Perforin “penetrated", apoptotic cell
(Also see color figure.) Antigen uptake and processing by nonimmune
system cells. Microbes and viruses can invade and infect any nucleated
cell, not only those involved in the immune response. The invading
organism is digested by lysosomal enzymes. Peptides encoded by infecting
DNA or RNA from the microbe are synthesized in the cell and then
transported to and displayed on MHC I molecules on the surface. The
MHC I displayed peptides are recognized by a T-cell receptor (TcR) on the
surface of cytotoxic T lymphocytes (CTLs). The infected cells are made
permeable by the protein perforin and destroyed by apoptosis.
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