Mature B cells generate protective immunity by undergoing immunoglobulin (Ig) class switching and somatic hypermutation, two Ig gene-diversifying processes that usually require cognate interactions with T cells that express CD40 ligand. This T-cell-dependent pathway provides immunological memory but is relatively slow to occur. Thus, it must be integrated with a faster, T-cell-independent pathway for B-cell activation through CD40 ligand-like molecules that are released by innate immune cells in response to microbial products. Here, we discuss recent advances in our understanding of the interplay between the innate immune system and B cells, particularly "frontline" B cells located in the marginal zone of the spleen and in the intestine.