Lymph Filtration in the Lymph Node occurs within interconnected lymphatic sinuses:
High Endothelial Venules contain lymphocytes circulating through the nodes. 10% enter the lymph node via the afferent lymphatic and 90% through the wall of the post-capillary venules. They are located in the deep cortex and are lined with cuboidal or columnar endothelium. They possess receptors for antigen-primed lymphocytes and signal the lymphocytes to leave circulate and migrate into the lymph node parenchyma. This allows resorption of some of the lymph directly into the bloodstream. B & T lymphocytes leave the bloodstream through postcapillary venules by diapedesis. They migrate between the endothelial cells. T lymphocytes stay in the thymus-dependent cortical zone (i.e. inner cortex) and the B lymphocytes migrate to the outer cortex. Lymphocytes leave lymph nodes via the lymphatic sinus, leading to efferent lymphatic vessels.
Lymph nodes serve as filters of lymph, where antigens are trapped by the reticular fibers and macrophages (the two make a meshwork in the sinuses) and by follicular dendritic cells (found in the germinal center of the lymphatic nodules). The antigens are processed by macrophages, dendritic cells, and B lymphocytes. This leads to the activation and differentiation of B lymphocytes. Activated B cells then migrate to the outer cortex, undergo mitosis (germinal center appears), and give rise to immature immunoblasts. These then give rise to plasma cells (which migrate to the medulla and emit antibodies into outgoing lymph) and memory B cells (which leave the node and circulate in the body, involved in the secondary response).