The immune response of broiler chickens exposed to intra-tracheal (i.t.) administration of benzo[a]pyrene (BaP) with and without nigella sativa (Ns) supplementation was investigated. A total of 120 day-old chicks were divided into four groups comprising 30 birds each, into a control, Ns, BaP, and BaP+Ns group. Immune responses to Newcastle disease (ND) were evaluated by haemagglutination inhibition (HI), phytohaemagglutinin (PHA) skin test and carbon clearance assay (CCA). In most instances, there was a significant increase (p<0.05) in the ND-HI antibody titers, PHA skin-swelling response and phagocytic activity in the BaP + Ns group compared to that of the BaP group. Likewise, organ weight and indices of the spleen, bursa of Fabricius and thymus of birds from the BaP + Ns group were also higher (p<0.05) than that of the BaP group from day 1 until day 21. It is concluded that exposure to BaP may exert adverse effects on the immune system of broilers which may increase their susceptibility to disease, and Ns supplementation significantly reduces these alterations.