The human immunoglobulin A Fc receptor FcαRI: a multifaceted regulator of mucosal immunity

Immunoglobulin A (IgA) is commonly recognized as the most prevalent antibody (Ab) at mucosal sites with an important role in defense by shielding mucosal surfaces from invasion by pathogens. However, its potential to both actively dampen excessive immune responses or to initiate potent proinflammatory cellular processes is less well known. Interestingly, either functional outcome is mediated through interaction with the myeloid IgA Fc receptor Fc alpha RI (CD89). Monomeric interaction of IgA with Fc alpha RI triggers inhibitory signals that block activation via other receptors, whereas multimeric Fc alpha RI crosslinking induces phagocytosis, reactive oxygen species production, antigen presentation, Ab-dependent cellular cytotoxicity, and cytokine release. Thus, Fc alpha RI acts as a regulator between anti-and proinflammatory responses of IgA. As such, the biology of Fc alpha RI, and its multifaceted role in immunity will be the focus of this review.