The mucosal immune system is present in anatomically and physiologically diverse tissues, including the gastrointestinal tract, nasopharynx, oral cavity, lung, eye, and urogenital tract. This chapter discusses the common and unique features of the mucosal immune systems in these organ tissues, focusing on the distribution of mucosal antibodies and immune inductive sites. In the oral cavity, mucosal and systemic Ig contributions depend on the state of oral health. In advanced periodontal disease, the proportion of plasma-derived IgG antibodies in the Ig pool in whole saliva increases substantially. The dominance of IgA in the upper respiratory tract and IgG in the lower tract is consistent with the mucosal immune system playing the major role in protection of the upper respiratory tract and the systemic immune system playing the main role in the lower tract. Similar to lower respiratory tract secretions, the dominant Ig isotype in secretions of the female and male genital tract and urine is IgG.