ABSTRACT
Whereas TI-1 antigens are inefficient inducers of affinity maturation and memory B cells, TI-2 antigens can induce both IgM and some class-switched responses. As we saw in Case 11, the initiation of B-cell class switching usually depends on the interaction of B cells and helper T cells via CD40 and CD40 ligand. Class switching in response to TI antigens is thought to involve other members of the TNF/TNFR family-the recently discovered TNF-like proteins BAFF (B-cell activating factor belonging to the TNF family) and APRIL (a proliferation-inducing TNF ligand), and their receptors on B cells. In human B cells, BAFF and APRIL induce class switching to IgA and IgG in the presence of TGF-b or IL-10 and to IgE in the presence of IL-4. In mice, in contrast, BAFF and APRIL on their own can switch B cells to IgG and IgA production.
