![]() The deepest structure within the lymph node is the medulla, consisting of cords of plasma cells and small B lymphocytes that facilitate immunoglobulin secretion into the exiting lymph. Follicles within the cortex are major sites of B-cell proliferation, whereas the interfollicular zone is the site of antigen-dependent T-cell differentiation and proliferation. Interior to the subcapsular sinus is the cortex, which contains primary follicles, secondary follicles, and the interfollicular zone. ![]() The sinuses are studded with macrophages, which remove 99% of all delivered antigens. This allows lymph, an ultrafiltrate of blood, to traverse from the afferent lymph vessels, through the sinuses, and out the efferent vessels. ![]() Directly interior to a lymph node's fibrous capsule is the subcapsular sinus. Lymph nodes, in conjunction with the spleen, tonsils, adenoids, and Peyer patches, are highly organized centers of immune cells that filter antigen from the extracellular fluid. Other, less common etiologies responsible for adenopathy include nodal accumulation of inflammatory cells in response to an infection in the node ( lymphadenitis), neoplastic lymphocytes or macrophages (lymphoma), or metabolite-laden macrophages in storage diseases ( Gaucher disease). Most lymphadenopathy in children is due to benign, self-limited disease such as a viral infection. Lymphadenopathy reflects disease involving the reticuloendothelial system, secondary to an increase in normal lymphocytes and macrophages (in response to an antigen).
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