What are the two signals required for class switching for CD4+ T cells, CD8+ T cells, and B cells?

What are the two signals required for class switching for CD4+ T cells, CD8+ T cells, and B cells?

• A. CD4+ T cell: 1) MHC-II with TCR/CD4 2) B7 with CD28; CD8+ T cell: 1) MHC-I with TCR/CD8 2) IL-2 from CD4+ binds to IL-2 receptor on CD8+; B cell: 1) IL-4, IL-5, IL-6 from Th2 2) CD40/CD40L interaction
• B. CD4+ T cell: 1) MHC-II with TCR/CD4 2) IL-2 receptor engagement; CD8+ T cell: 1) MHC-I with TCR/CD8 2) CD40/CD40L interaction; B cell: 1) IL-10 2) IL-12
• C. CD4+ T cell: 1) MHC-II with TCR/CD4 2) B7 with CD28; CD8+ T cell: 1) MHC-I with TCR/CD8 2) IL-2 from CD4+ binds to IL-2 receptor on CD8+; B cell: 1) IL-4, IL-5, IL-6 from Th2 2) CD40/CD40L interaction
• D. CD4+ T cell: 1) MHC-II with TCR/CD4 2) CD40/CD40L; CD8+ T cell: 1) MHC-I with TCR/CD8 2) IFN-γ; B cell: 1) CD40 with CD28 2) IL-4

Answer: (C)

Class switching relies on two different signals delivered to each cell type: a primary signal that recognizes antigen, and a second signal that provides the necessary activation cue from interacting cells. For CD4+ T cells, the first signal comes from the TCR recognizing peptide presented on MHC class II, with the CD4 co-receptor reinforcing that interaction. The second signal is delivered by a costimulatory interaction, B7 on the APC binding CD28 on the T cell, which fully activates the helper T cell to support other immune responses. For CD8+ T cells, the first signal is peptide-MHC class I recognition through the TCR in cooperation with CD8. The second signal is a growth-promoting cue provided by IL-2 produced by helper T cells, which binds to the IL-2 receptor on the CD8+ T cell to drive proliferation and differentiation into cytotoxic T cells. For B cells, class switching is guided by helper T-cell signals: cytokines such as IL-4, IL-5, and IL-6 steer the isotype outcome, while CD40 on the B cell engaging CD40L on the helper T cell provides essential co-stimulation that enables the B cell to undergo isotype switching. This combination of antigen-driven signaling plus T-cell–derived costimulation and cytokine cues explains why the described pairings best fit the requirement for class switching across these cell types.

Class switching relies on two different signals delivered to each cell type: a primary signal that recognizes antigen, and a second signal that provides the necessary activation cue from interacting cells.

For CD4+ T cells, the first signal comes from the TCR recognizing peptide presented on MHC class II, with the CD4 co-receptor reinforcing that interaction. The second signal is delivered by a costimulatory interaction, B7 on the APC binding CD28 on the T cell, which fully activates the helper T cell to support other immune responses.

For CD8+ T cells, the first signal is peptide-MHC class I recognition through the TCR in cooperation with CD8. The second signal is a growth-promoting cue provided by IL-2 produced by helper T cells, which binds to the IL-2 receptor on the CD8+ T cell to drive proliferation and differentiation into cytotoxic T cells.

For B cells, class switching is guided by helper T-cell signals: cytokines such as IL-4, IL-5, and IL-6 steer the isotype outcome, while CD40 on the B cell engaging CD40L on the helper T cell provides essential co-stimulation that enables the B cell to undergo isotype switching.

This combination of antigen-driven signaling plus T-cell–derived costimulation and cytokine cues explains why the described pairings best fit the requirement for class switching across these cell types.