Clusters of lymphoid cells beneath the mucosal endothelium of the gastrointestinal, respiratory, and genitourinary tracts that help protect the body from inhaled, consumed, or sexually transmitted infections. The system has two parts: organized and diffuse. The organized part (the mucosal-associated lymphoid tissue of the gastrointestinal and respiratory tracts) is composed of nodules containing lymphocytes and macrophages that are activated by ingested or inhaled microorganisms. The diffuse part is composed of loose clusters of macrophages and mature B and T lymphocytes found within the folds of the intestinal walls. The B cells secrete antibodies, primarily immunoglobulin A; the T cells directly lyse microorganisms.