periarteriolar lymphatic sheaths
Summary
The periarteriolar lymphatic sheath (PALS) is the T-cell-rich region of the white pulp of the spleen, surrounding the central arterioles. It is the site where antigen-presenting cells present antigens to T lymphocytes, initiating cell-mediated immune responses.
Detail
The spleen's white pulp consists of lymphoid tissue organized around central arterioles, and the PALS forms the innermost layer of this tissue. It is composed predominantly of CD4+ and CD8+ T lymphocytes, along with interdigitating dendritic cells that present antigens carried by the blood. Adjacent to the PALS are B-cell-containing lymphoid follicles (which may develop germinal centers upon antigen stimulation), and further outward lies the marginal zone, which contains macrophages and B cells that filter blood-borne antigens and pathogens, particularly encapsulated bacteria. Functionally, the PALS is analogous to the paracortex of lymph nodes, serving as the site of T-cell activation and proliferation in response to blood-borne antigens. Clinically, the spleen's white pulp (including PALS) plays a crucial role in mounting immune responses to blood-borne pathogens, especially encapsulated organisms like Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis. This underlies the increased susceptibility to these infections in asplenic patients (e.g., post-splenectomy, sickle cell disease with autosplenectomy), necessitating vaccination against these encapsulated bacteria. Histologically, the spleen is often tested via identification of white pulp (PALS + follicles) versus red pulp (venous sinuses and cords of Billroth, involved in filtering senescent RBCs and housing macrophages).
Sources
- Robbins and Cotran Pathologic Basis of Disease
- Junqueira's Basic Histology
- First Aid for the USMLE Step 1
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