Final answer:
CD28 and CD40 are the CD antigens that serve as receptors for the co-stimulatory signal required for B cell activation.
Step-by-step explanation:
The CD antigens that are receptors for a co-stimulatory (secondary) signal for B cells are CD28 and CD40. During the activation process, a B cell must receive two signals. The first is when its surface immunoglobulin (B-cell Receptor or BCR) recognizes the native protein antigen, which is then internalized and presented on a class II MHC molecule to Th2 cells.
The second, co-stimulatory signal is provided by the interaction between CD40 on the B cell and CD40L on the Th2 cell, and also by the interaction of CD28 on the T cell with its ligands CD80/86 on the B cell. These interactions are critical for the full activation and proliferation of B cells, leading to an effective immune response.