Expansion
Cortex with follicles, paracortex with T cells, and medulla with plasma cells
Expansion
Cortex
- Primary follicles: resting B cells
- Secondary follicles with germinal centres: activated B cells undergoing proliferation, somatic hypermutation and class switching
Paracortex
- T cells and interdigitating dendritic cells
- Contains high endothelial venules, through which lymphocytes enter from blood
- Poorly developed in DiGeorge syndrome
Medulla
- Medullary cords: plasma cells and macrophages
- Medullary sinuses draining to the efferent lymphatic
Flow: afferent lymphatics enter through the capsule into the subcapsular sinus, then through cortical and medullary sinuses, leaving by a single efferent lymphatic at the hilum.
Reactive patterns are diagnostically useful:
- Follicular hyperplasia: B cell response, as in bacterial infection, rheumatoid arthritis and early HIV
- Paracortical hyperplasia: T cell response, as in viral infection and after vaccination
- Sinus histiocytosis: nodes draining a tumour
- Granulomas: tuberculosis, sarcoidosis