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chapter 35

Molecular Immunology

FIGURE 35-1

(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.

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FIGURE 35-2

(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

FIGURE 35-3

(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.