Oral medicine: 10. Lumps and swellings: neck.

The lymphoid system is the essential basis of immune defences and comprises predominantly bone marrow, spleen, thymus and lymph nodes. Tissue fluid drains into lymph nodes which act as ‘filters’ of antigens and, after processing in the nodes, lymph containing various immunocytes drains from the nodes to lymph ducts and then to the circulation. A lymph node consists of a cortex, paracortex and medulla and is enclosed by a capsule. Lymphocytes and antigens (if present) pass into the node through the afferent lymphatics, are ‘filtered’, and pass out from the medulla through the efferent lymphatics. The cortex contains B cells aggregated into primary follicles; following stimulation by antigen these develop a focus of active proliferation (germinal centre) and are termed secondary follicles. These follicles are in intimate contact with antigen-presenting dendritic cells. The paracortex contains T cells, and the medulla contains T and B cells.