The effect of age on peripheral blood CD56bright and CD56dim NK cell number. These graphs illustrate (a) the number of CD56dim NK cells μl/blood and (b) the percentage of CD56bright NK cells μl/blood within samples in each age group; 20–40(n = 25) 40–60(n = 23) and 60 + (n = 67). The mean values within each respective cohort group are displayed. Statistical analysis was carried out as described in the Methods and p values between groups are shown. These results are summarised together with the percentage of the lymphoid pool occupied by CD56bright and CD56dim cells and are displayed in table 2.