Defensins are small microbial peptides that are classified on the basis of their size and pattern of disulfide bonding into alpha, beta and theta categories. They are produced constitutively or in response to microbial products or proinflammatory cytokines. Denfins are small, cystein-rich peptides with antimcrobial activity.

The anti-microbial mechanism(s) of defensins is not fully understood, but it is generally believe that target killing is a consequence of disruption of the microbial membrane.

Alpha-defensins are abundant in neutrophils, macrophages, and Peneth cells of the small intestine, but are also produced by cytokine-stimulated NK cells, and B cells. Alpha-defensins have recently been shown to inhibit in vitro HIV infection.

Alpha-defensins are cyclic and are only prodcued in primary phagocytes.

The conservation of defensins in animals and plants reveals the acient origins of innate immunity. The exact mechanisms are being worked out, but in some cases plant defensins inhibit protein syntesis. Plants are more susceptible to bacterial and fungal infections when the expression of defensin genes is supressed.