Effect of dietary propolis (PR) supplementation on (a) T-lymphocyte proliferation, and (b) B-lymphocyte proliferation of broiler chickens challenged with Escherichia coli (EC) infection. Bars express the mean of stimulation indexes (SI) for T- or B-lymphocytes (n = 20) with standard error (SE). Means with dissimilar letters significantly differ at p < 0.05. Treatment groups: C, control group without PR supplementation or EC challenge; EC, group without PR supplementation but challenged with EC; PR, group supplemented with PR but not challenged with EC; PR+EC, group supplemented with PR and challenged with EC.