The composite nodule. A structural and functional unit of the reactive human lymph node.

The relationship between T nodules and adjacent B-lymphoid follicles was investigated in 37 reactive lymph nodes by light microscopy and combined enzyme immunohistochemistry. In 16 cases (43%), T nodules and adjacent B-lymphoid follicles were unified in an ovoid, distinct nodular structure termed a "composite nodule." The composite nodule comprises two separate domains. The peripheral, subcapsular B domain contains all stationary and migratory elements of the B-lymphoid follicle, ie, B1+ B-cells, OKT4+, Leu 3a+ helper/inducer T cells, HLA-DR+ dendritic reticulum cells, and ANAE+, AcPhase+ tangible body macrophages and is surrounded by a B1+, HLA-DR+ lymphocytic corona displaying focal adenosine triphosphatase (ATPase) and alkaline phosphatase (AlkPhase) activity. The deep, paracortical T-domain contains all elements of the T nodule, ie, OKT4+, Leu3a+ helper/inducer T cells, high endothelial venules and HLA-DR+, ATPase+ interdigitating reticulum cells. The composite nodule is surrounded by a rim of ATPase+, AlkPhase+ high endothelial venules. Both domains are subject to changes in volume; thus, in follicular hyperplasia, the B domain enlarges at the cost of the T domain, and the reverse may occur in T-zone hyperplasia. Based on the striking resemblance between the composite nodule and the white pulp of the spleen, it is suggested that the composite nodule plays a major role in the triggering, helper-T-cell-dependent stimulation and subsequent maturation of antigen-responsive B cells into antibody-secreting plasma cells.