CD40 deficiency affects CD73 expression on age‐associated B cells (ABCs) and peripheral T cells. Frequency of CD73+ cells on gated blood neutrophils (a, n = 10), peritoneal cavity B‐1 B cells (b, n = 10), splenic regulatory T (Treg) cells (c, n = 10), marginal zone (MZ) B cells (d, n = 14), ABCs (e, n = 15), B regulatory (Breg) cells (f, n = 12), Peyer's patch follicular helper T (Tfh) (g, n = 8) and splenic CD4+ T (h, n = 9) and CD8+ T cells (i, n = 9) in BALB/c and CD40−/− mice. (j, k) Frequency of CD73+ cells on gated CD4+ and CD8+ (n = 8) T cells in B6 and μMT mice. Data are shown as mean ± SEM of 8–15 mice, as indicated, from two to five experiments. *P ≤ 2·4 × 10−9, **P = 0·01.