B-cell subsets and the mature preimmune repertoire. Marginal zone and B1 B cells as part of a "natural immune memory".

The rate of elimination of a pathogenic agent is of critical importance for the host and determines the extent and consequences of the infection. Antibody production, along with the activity of other cells of the immune system, plays an important role early and late in the response and contributes to all containment and elimination of the organism. B-cell clones reaching the mature long-lived pool are heterogeneous: some belong to the B1 B-cell subset, some are enriched in the CD21high compartment (mostly marginal zone (MZ)), whereas others recirculate primarily among the B-cell follicles (FO). This segregation is a T-independent, CD40L-independent but BCR/CD19-dependent process. Antigen encounter will recruit antigen-specific cells from the pool of mature B-lymphocytes and activate them to perform effector functions. CD21highCD23low B cells enriched in the MZ of the spleen initiate the early plasmablast wave during the first 3 days of an antibody response against particulate T-independent bacterial antigens. These findings indicate a functional heterogeneity within the mature B-lymphocyte population. MZ B cells and B1 B, in contrast to FO B cells, have the unique capacity to generate effector cells in early stages of the immune response against (particulate) antigens that are scavenged efficiently in these specialized anatomical sites.