........

e.g., CD3,
CD4, and CD8. CD4 is a coreceptor, a rod-shaped
single polypeptide with an extracellular part
consisting of four immunoglobulinlike
domains. The gp120 protein of the HIV virus reacts
with the second domain of CD4. A number
of accessorymembrane proteins of the CD3 system
participate in the specific binding between
TCR and MHC (TCR–CD3 complex). (Drawings
adapted from Abbas et al., 1997.)
The figures in B are highly schematic and do not
show the real three-dimensional structures.