CD27 and influenza: The kinetics of other immune response (flow cytometry) variables relative to vaccination (e.g., overall IgD+CD27- B cells, IgD+CD27-/naive B cells, IgD+CD27-/transitional B cells, etc.), and the observed correlations between these variables and influenza vaccine-specific immune outcomes in our study, suggest the involvement of other B cell populations and immune response mechanisms (e.g., de novo antibody response) in the overall response after immunization in older individuals.