Immunotoxic response can originate from direct toxicity of the pharmaceutical to components of the immune system. Direct immunotoxicity may result in either immunosuppression or immunostimulation. Alternatively, the immune system may react to chemicals in an antigen-specific way. Therefore, immunotoxicology is an important aspect of the safety evaluation of drugs and chemicals. Despite the remarkable success of rodent-based models in determining immunotoxicity, there are limitations in extrapolating the risk posed to humans. Immunotoxicological data resources derived from studies using human primary leukocytes can facilitate rational decision making for drug development.