Both CD28 and CTLA-4 Are Receptors on T Cells That

Multiple Choice

Both CD28 and CTLA-4 are receptors on T cells that are critical for regulating T cell activation.In which one of the following ways does CD28 differ from CTLA-4?

A) Only CD28 binds the costimulatory ligands B7-1 and B7-2 expressed on professional antigen-presenting cells.
B) CD28 counteracts positive,pro-proliferative T cell signals delivered by CTLA-4.
C) CD28 is constitutively expressed on naive T cells,whereas CTLA-4 is expressed on activated T cells.
D) CD28 binds its ligand with 10-fold greater affinity than does CTLA-4.
E) CD28 is important for delivering "signal 1" for T cell activation,whereas CTLA-4 is important for delivering "signal 2."