Marginal zone B cells: virtues of innate-like antibody-producing lymphocytes

Marginal zone B cells are strategically positioned at the interface between the circulation and the white pulp of the spleen, where they can mediate a rapid, innate-like humoral response against both pathogens and commensals that breach the mucosal barrier. Understanding the interplay of these cells with other immune components and the species-specific differences between mice and humans is of clinical importance. Protective responses to microorganisms involve the nonspecific but rapid defence mechanisms of the innate immune system, followed by the specific but slow defence mechanisms of the adaptive immune system. Located as sentinels at the interface between the circulation and lymphoid tissue, splenic marginal zone B cells rapidly respond to blood-borne antigens by adopting 'crossover' defensive strategies that blur the conventional boundaries of innate and adaptive immunity. This Review discusses how marginal zone B cells function as innate-like lymphocytes that mount rapid antibody responses to both T cell-dependent and T cell-independent antigens. These responses require the integration of activation signals from germline-encoded and somatically recombined receptors for microorganisms with helper signals from effector cells of the innate and adaptive immune systems.