Abstract The immune system protects animals from the effects of pathogens including bacteria and viruses, hence it plays an important role in their survival. However, immune function is affected greatly by food shortage. Our previous research showed that 3 days of fasting suppressed phytohaemagglutinin (PHA) response in female Mongolian gerbils (Meriones unguiculatus). The duration of fasting might play a key role in influencing animals’ immunity. In order to clarify this question, we investigated how different fasting durations would have effects on immunity in gerbils. Fifty-five adult female gerbils were randomly divided into 4 groups that, in turn, were subjected to 1 day, 2 days, and 3 days of fasting or fed ad libitum, respectively. Half of each treatment group received phosphate buffered saline or PHA injection. We found that PHA response indicative of T-cell-mediated immunity was suppressed gradually with duration of fasting. Similarly, fasting led to spleen and thymus atrophy in gerbils. These results indicate that the duration of fasting had an important effect on immunity. Moreover, body fat mass, blood glucose and leptin levels decreased gradually with duration of starvation, which were all positively correlated with PHA response. However, white blood cells and corticosterone concentrations were not affected by fasting. Taken together, the duration of fasting had an important effect on immunity in gerbils. Suppression of T-cell-mediated immunity might be due to the gradual decrease of body fat mass, blood glucose and leptin levels with the time of fasting.