Patients with Down syndrome carry immunologic defects, as evidenced by the increased risks for autoimmune diseases, hematologic malignancies, and respiratory tract infections. Moreover, the low numbers of circulating B cells suggest impaired humoral immunity.

We sought to study how immunodeficiency in patients with Down syndrome results from immunologic defects in the B-cell compartment.

We studied blood B-cell subset composition, replication history, somatic hypermutation status, and class-switch recombination in 17 children with Down syndrome. Germinal centers and plasma cells were studied in tonsils from 4 additional children with Down syndrome.

Blood transitional B-cell numbers were normal, but naive mature and memory B-cell numbers were reduced despite slightly increased serum B cell–activating factor levels. Germinal centers and plasma cells in tonsils appeared normal, as were serum immunoglobulin levels. CD27⁺IgD⁺IgM⁺ “natural effector” B cells showed reduced proliferation and somatic hypermutation levels, whereas these were normal in CD27⁺IgD⁻ memory B cells. Furthermore, IgM⁺ and IgA⁺, but not IgG⁺, memory B cells showed impaired molecular signs for antigen selection. The B-cell pattern was highly similar to that of patients with common variable immunodeficiency and a defect in B-cell activation and proliferation.

Children with Down syndrome seem capable of normal germinal center and plasma cell formation. Still, blood memory B-cell numbers were reduced and showed impaired molecular maturation of IgA and IgM, which are important for mucosal immunity. The observed molecular defects in circulating IgA and IgM B-cell memory could reflect impaired local responses, which underlie the increased susceptibility to respiratory tract infections of patients with Down syndrome.