Which interaction is essential for T cell help to B cells and enables antibody class switching?

Which interaction is essential for T cell help to B cells and enables antibody class switching?

• A. CD40L on T helper cells binding CD40 on B cells
• B. CD28 with B7
• C. MHC-TCR interaction
• D. IL-2 receptor activation

Answer: (A)

The essential driver of T cell help to B cells for antibody class switching is the direct interaction between the CD40 ligand on helper T cells and the CD40 receptor on B cells. When a helper T cell engages a B cell presenting antigen, it upregulates CD40L and binds CD40 on the B cell. This “signal 2” delivered through CD40-CD40L is crucial for B cell proliferation, survival, and entry into germinal centers, where class switch recombination occurs. This interaction also promotes the expression of activation-induced cytidine deaminase (AID) in B cells, the enzyme required for class switch recombination and somatic hypermutation, enabling B cells to switch from producing IgM to other isotypes such as IgG, IgA, or IgE. Without CD40-CD40L signaling, B cells fail to undergo CSR and proper affinity maturation, even if initial B cell receptor engagement and T cell activation occur. Other interactions—like CD28 with B7, which mainly provides co-stimulation for T cell activation, MHC-TCR engagement for antigen recognition, or IL-2 receptor signaling for T cell growth—do not by themselves drive the essential B cell CSR process.

The essential driver of T cell help to B cells for antibody class switching is the direct interaction between the CD40 ligand on helper T cells and the CD40 receptor on B cells. When a helper T cell engages a B cell presenting antigen, it upregulates CD40L and binds CD40 on the B cell. This “signal 2” delivered through CD40-CD40L is crucial for B cell proliferation, survival, and entry into germinal centers, where class switch recombination occurs.

This interaction also promotes the expression of activation-induced cytidine deaminase (AID) in B cells, the enzyme required for class switch recombination and somatic hypermutation, enabling B cells to switch from producing IgM to other isotypes such as IgG, IgA, or IgE. Without CD40-CD40L signaling, B cells fail to undergo CSR and proper affinity maturation, even if initial B cell receptor engagement and T cell activation occur.

Other interactions—like CD28 with B7, which mainly provides co-stimulation for T cell activation, MHC-TCR engagement for antigen recognition, or IL-2 receptor signaling for T cell growth—do not by themselves drive the essential B cell CSR process.