a The intestinal epithelium comprises several cell lineages. Enterocytes constitute the most abundant epithelial cell type, and secrete several antimicrobial proteins (AMPs) such as regenerating islet-derived protein 3γ (REG3γ). Paneth cells are unique to the small intestine and secrete abundant quantities of AMPs, such as α-defensins. Finally, goblet cells secrete mucin glycoproteins that assemble to form a thick mucus layer overlying the epithelium. The mucus layer seems to have a crucial role in concentrating secreted AMPs near the epithelial surface. b The epithelial barrier of the skin includes keratinocytes at the surface and the hair unit, and specialized secretory organs such as sebocytes and eccrine glands. Many diverse AMPs, including cathelicidins and defensins, are produced by these cells under steady-state and/or inflammatory conditions. The aqueous and lipid components of the skin surface combine with AMPs produced by microorganisms to enhance the barrier/protective function. The aqueous/lipid layer may serve a function that is similar to that of intestinal mucus by trapping AMPs at the epithelial surface. Resident bone marrow-derived cells in the dermis, such as mast cells, provide essential additional AMPs after skin injury or in early stages of infection. αMSH, α-melanocyte-stimulating hormone; ANG4, angiogenin 4; BD, β-defensin; PSM, phenol-soluble modulin; RNase7, ribonuclease 7; SLPI, secretory leukocyte protease inhibitor (also known as ALP).