A helper T cell response to a protein antigen requires the participation of antigen-presenting cells that express which of the following types of molecules?
A. Class II MHC and costimulators
B. Class I MHC and CD4
C. Class II MHC and CD8
D. CD4 and costimulators
E. Class II MHC and CD4
ANS: A. Helper T cells are almost always CD4+. The activation of naive CD4+ T cells requires T cell receptor recognition of class II MHC-peptide complexes and the binding of costimulators, both on the antigen-presenting cell (APC) surface. CD4+ helper T cells bind to class II MHC molecules on the APC, not to class I MHC molecules. CD4 or CD8 expression on the APC surface is of no known relevance to T cell activation.
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