marginal zone

Anatomically, the spleen's white pulp consists of the PALS (T-cell zone surrounding central arterioles), lymphoid follicles (B-cell zone with germinal centers), and the marginal zone, which lies at the interface between white pulp and red pulp. The marginal zone contains marginal zone B cells—a distinct subset of memory-like B cells capable of rapid, T-cell-independent antibody responses to blood-borne antigens, particularly encapsulated bacteria (polysaccharide antigens). This makes the marginal zone critical for early humoral defense against pathogens like Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis—explaining why asplenic patients (post-splenectomy or functional asplenia, e.g., sickle cell disease) are at increased risk for infections with encapsulated organisms and require vaccination (pneumococcal, meningococcal, Hib) and sometimes prophylactic antibiotics.

The marginal zone also filters blood, allowing macrophages to capture antigens, opsonized particles, and damaged/senescent red blood cells for removal.

Clinically, marginal zone lymphoma (MZL) arises from these B cells and is a low-grade non-Hodgkin lymphoma. Three main subtypes exist: extranodal MALT lymphoma (mucosa-associated lymphoid tissue, classically gastric, associated with H. pylori infection—treatment involves H. pylori eradication in early stages), splenic marginal zone lymphoma (associated with hepatitis C infection), and nodal marginal zone lymphoma. These lymphomas are often associated with chronic inflammation or autoimmune conditions (e.g., Sjögren syndrome leading to salivary gland MALT lymphoma, Hashimoto thyroiditis leading to thyroid MALT lymphoma). Histologically, they show a heterogeneous population of small lymphocytes, and can show lymphoepithelial lesions in MALT variants. Molecular findings may include t(11;18) translocation in MALT lymphoma, associated with API2-MALT1 fusion, which confers resistance to H. pylori eradication therapy alone.