Other lymphoid tissue

Overview

Beyond lymph nodes, the immune system includes several other organised and diffuse lymphoid structures - the spleen, thymus, and mucosa-associated lymphoid tissue (MALT) - each with a distinct role
Splenectomy (surgical or functional, e.g. in sickle cell disease) carries a well-defined, lifelong increased risk of overwhelming infection with encapsulated organisms

Spleen: white pulp (lymphoid tissue - immune surveillance of blood-borne antigen, analogous to a lymph node for the bloodstream) and red pulp (filters aged/abnormal red cells, stores platelets, extramedullary hematopoiesis if needed)
Thymus: site of T cell maturation and central tolerance (positive and negative selection) - involutes with age, contributing to age-related decline in naive T cell output (immunosenescence)
MALT (gut-associated - Peyer's patches, and similar tissue in the respiratory/urogenital tract): first-line mucosal immune surveillance, particularly important for IgA production at mucosal surfaces

Post-splenectomy blood film: Howell-Jolly bodies (nuclear remnants normally removed by the spleen) are the classic finding indicating loss of splenic function, whether from surgical splenectomy or functional hyposplenism

Asplenic/hyposplenic patients require: vaccination (pneumococcal, meningococcal, Hib - the organisms the spleen is most critical for clearing), prophylactic antibiotics (especially in the first 2 years post-splenectomy and in children), and patient education to seek urgent medical review for any febrile illness (risk of overwhelming post-splenectomy infection, OPSI)
Thymectomy is a treatment consideration in myasthenia gravis, particularly with a thymoma or in younger patients with generalised disease

Overwhelming post-splenectomy infection (encapsulated organisms - Streptococcus pneumoniae especially), sickle cell disease (functional asplenia from recurrent splenic infarction), myasthenia gravis/thymoma, coeliac disease (MALT/gut immune dysregulation)

OPSI carries a high mortality if not treated as an emergency - any asplenic/hyposplenic patient with fever needs urgent assessment and empirical antibiotics, not a "wait and see" approach

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