Secondary lymphoid organs

The lymph node is a small, bean-shaped structure and usually the very first site where an antigen entering the tissue spaces is intercepted. Hundreds of lymph nodes are strung throughout the body, linked by lymph vessels, and each is packed with lymphocytes and macrophages that filter and trap material carried in the lymph. Structurally, a lymph node has three zones: the outer cortex, rich in B lymphocytes, macrophages and follicular dendritic cells; the underlying paracortex, dominated by T lymphocytes and interdigitating dendritic cells; and the innermost medulla, sparsely populated but containing plasma cells that actively secrete antibody. As lymph percolates through these three zones in turn, phagocytic and dendritic cells get repeated opportunities to trap antigen, and the lymph that finally leaves the node carries antibody enriched by the medulla's plasma cells. Because active immune responses draw in extra lymphocytes, an infected region's lymph nodes (commonly in the neck, under the chin, in the armpits or in the groin) can visibly swell — a familiar clinical sign of infection. …