Abstract
Mature B cells occupy three compartments: (1) recirculating between the follicles of secondary lymphoid tissues; (2) in the pleural and peritoneal cavities; and (3) non-recirculating marginal zone (MZ) B cells reside at the junction between the splenic white and red pulp, where they are ideally situated to pick up blood-borne antigens (Ags). MZ B cells appear critical for antibody responses to the polysaccharide capsules of pathogenic bacteria including Streptococcus pneumonia and Haemophilus influenzae B. IgG antibody protects against these pathogens and responses only develop in infants after the age of two years, coincident with maturation of the MZ. Also, adult splenectomy results in increased susceptibility to infection with the same organisms. Polysaccharide Ags can induce responses in the absence of MHC class II-restricted T-cell help and are therefore described as T-independent (TI), specifically as TI type 2 (TI-2) to distinguish them from lipopolysaccharides (TI-1), which are mitogenic for B cells. In vitro, MZ B cells differentiate more rapidly into antibody-forming cells than do their follicular counterparts, they are therefore well equipped and strategically placed to provide the first line of defence in vivo, not only against TI-2 but other Ags.
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