paracortex

The paracortex is a region of the lymph node located between the cortex and the medulla. It is commonly referred to as the T cell zone because it is rich in T lymphocytes and antigen-presenting cells, particularly interdigitating dendritic cells. The cortex contains B cell follicles, while the paracortex harbors few organized follicles unless a germinal center forms during an ongoing response. The area features a high density of high endothelial venules, through which naïve T cells enter the lymph node from the bloodstream to participate in immune surveillance.

Functionally, the paracortex is the primary site for T cell activation within the lymph node. Antigen-presenting dendritic cells migrate into this region and present processed antigens to naïve T cells, promoting their activation, clonal expansion, and differentiation. Activated T cells then exit the paracortex to exert effector functions or assist B cells in the cortex during humoral responses.

Clinical notes include paracortical hyperplasia, an expansion of this zone observed in various conditions, notably certain viral infections such as infectious mononucleosis, and in some immune reactions. Lymphomas, including T cell lymphomas and some non-Hodgkin lymphomas, can involve the paracortex.

In summary, the paracortex is the T cell–rich part of the lymph node, essential for cellular immune activation and the coordination of adaptive immune responses within this secondary lymphoid organ.