B cell phenotypes. The phenotype of B lymphocytes that respond to vaccination with Pneumovax23® was determined by flow cytometry. Pre-vaccination (n=9) and seven days post-vaccination (n=18) circulating PBMC were isolated and labeled for analysis using the following fluorescently labeled antibodies/antigen: CD19, CD27, IgM, IgD, PPS14 and PPS23F. The phenotype of pre-vaccination unselected B cells were compared to PPS-specific B cells (A), the phenotype of post-vaccination unselected B cells were compared to PPS-specific B cells (B) and pre-immunization PPS-specific B cells were compared to post-immunization PPS-specific B cells (C). In each sample 75,000 events were recorded. *p<0.0001 **p=0.0002 #p=0.037 ##p=0.006