Figure 1. M cells and the induction of mucosal immunity. M cells are present in mucosal inductive sites in both the intestinal and upper respiratory tract, specifically in Peyers patches and the nasal-associated lymphoid tissue, the tonsils and adenoids. M cells are thought to play an important role in antigen processing and possibly the induction of antigen-specific mucosal immunity in mucosal effector sites. Sites followed by question marks are presumed sites since limited data are available on these sites.