Just as potentially useful T cells are positively selected by MHC-peptide complexes in the thymus, it has been proposed that self or commensal bacterial epitopes might select B cell populations with the capacity to recognize polysaccharide or protein structures on pathogens. Recent studies indicate that the repertoire of 8 cells entering the periphery is not shaped by specific stimuli, but that mature B cell subsets may be under different selective pressures.
Weill, J. C., & Reynaud, C. A. (2005, January 3). Do developing B cells need antigen? Journal of Experimental Medicine. https://doi.org/10.1084/jem.20042111