Age-related changes in the cellular composition of the peritoneal lavage (PLF), spleen, and blood of naive (non-immunized) BALB/c mice. Upper panels (A-D) show the changes of the T cell repertoire: (A)T cell percentage, (B) CD44high (memory): CD62Lhigh (naive) CD4+ T cell ratio, (C) CD28 expression on CD4+ T cells, and (D) Treg ratio. Lower panels (E-H) summarize the age-related changes in APC composition in the PLF and spleen: (E) conventional B cells, (F) B1 cells (#In the spleen this cell population is comprised of B1 cells, transitional B cells and marginal zone B cells), (G) macrophages (F4/80+ cells), and (H) dendritic cells (CD11c+ cells). Data represent the mean ± SEM (n = 4–5 mice per group). Values were significantly (p < 0.05) *lower or †higher than those measured in the 1-month-old group as indicated.