Single lymphoid nodules (noduli lymphoidei solitarii) are found in the thickness of the mucosa and in the submucosa of the digestive system (pharynx, esophagus, stomach, small intestine, large intestine, gallbladder), respiratory organs (larynx, trachea, major, lobar and segmental bronchi ), as well as in the walls of the ureters, bladder, urethra.