paracortex

**Location and structure (lymph node):** - Lymph node layers from outside in: capsule → subcapsular sinus → superficial cortex (B-cell follicles: primary and secondary with germinal centers) → **paracortex** → medulla (medullary cords with plasma cells and macrophages, and medullary sinuses). - The paracortex lies between the follicles of the cortex and the medulla. It is sometimes called the deep cortex or thymus-dependent zone.

**Cellular composition:** - Mostly naive T lymphocytes (CD4+ helper and CD8+ cytotoxic). - Interdigitating dendritic cells, which present antigen on MHC to T cells after migrating from peripheral tissues (for example Langerhans cells from skin) through afferent lymphatics. - **High endothelial venules (HEVs)**: cuboidal endothelium where naive T and B cells enter the node from blood. Entry uses L-selectin (CD62L) on lymphocytes binding GlyCAM-1/CD34 on HEVs, then CCR7 signaling and LFA-1/ICAM-1 for firm adhesion and diapedesis. CCL19 and CCL21 attract CCR7+ T cells and dendritic cells into this zone.

**Function:** - Site of T-cell priming. Antigen-bearing dendritic cells activate naive T cells here. - Activated CD4+ T cells interact with B cells at the follicle border to support germinal center formation (class switching, affinity maturation). - Activated T cells exit through efferent lymphatics.

**Clinical correlations (high-yield):** - **Paracortex hypoplasia or depletion:** DiGeorge syndrome (22q11.2 deletion, thymic aplasia) and other T-cell deficiencies such as SCID. The paracortex of lymph nodes and the periarteriolar lymphatic sheath (PALS) of the spleen are both depleted. - **Paracortex expansion (paracortical hyperplasia):** viral infections (EBV/infectious mononucleosis, CMV), some drug reactions (for example phenytoin), vaccinations, and other T-cell-mediated responses. Paracortical expansion is also seen in dermatopathic lymphadenitis. - **Follicular (cortical) hyperplasia** is the contrast, seen with B-cell activation such as rheumatoid arthritis, early HIV, and syphilis. - **Spleen analogue:** the PALS (periarteriolar lymphoid sheath) around the central arteriole in white pulp holds T cells, as the paracortex does in the node. - **Other T-cell zones:** thymic medulla and Peyer patch interfollicular regions. - **Memory aid:** "Paracortex = T cells (para = beside the B-cell follicles); Follicles = B cells; Medulla = plasma cells (antibody secretion)."

**Test-taking tip:** Questions often describe a child with hypocalcemia, a cardiac defect, and recurrent viral or fungal infections, then ask which lymph node region is depleted. The answer is the paracortex.