What are lymphoid nodules and where are they located? What functions are served by these? (1 paragraph) Solution These are small localised ovoid or kidney shaped organs containing collection of lymphoid tissue. They are generally located in the loose connective tissue under wet epithelial membranes eg-digestive system, respiratory system etc. They are located in the walls of alimentary canal (tonsils, appendix and caecum, Peyer\'s patches), genitourinary passages, respiratory tract and also in other parts of the body. Peyer\'s patches are permanently localized lymphoid tissue in the sub mucosa of ileum. To defend nasal and oral cavities in the body lymphatic nodules are grouped together forming tonsils which are of three pairs viz. Palatine, Lingual and Pharyngeal. They help in the proper functioning of the immune system. They act as a filter for foreign particles and cancer cells. They guard the body and respond to entry of infectious organisms and toxins. Lymphoid nodules are the sites of proliferation/activation of lymphocytes (B cells). It occurs in the germinal centre..