CHA PTER 35, FIG U R E 1 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 major histocompatibility complex
(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 synthe-
size and secrete antibodies.
Perforin "p m rcraled . apoplotlc cell
CHA PTER 35, FIG URE 2 Antigen-antibody complex formation
and recognition by macrophages. Antigens that react directly with anti-
bodies 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 anti-
gen-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.
CHA PTER 35, FIG URE 3 Antigen uptake and processing by nonim-
mune 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 cytotox-
ic T lymphocytes (CTLs). The infected cells are made permeable by the pro-
tein perforin produced by the CTLs and destroyed by apoptosis.