Introduction: Respiratory Disease Complex in broilers is a major disease in poultry production, which is caused by respiratory pathogens, including IBV, NDV, AIV H9N2, ART, MG, MS, E. coli and ORT. Insufficient control of Environmental condition in houses, genetic of the commercial hybrid strains, stressful events associated to hatch, transportation, stress of production have also big impact in respiratory disease complex. Immunosuppression caused by IBDV, MDV, CIAV, astrovirus, reovirus, parvovirus and parvovirus, along with mycotoxins are also among major players of this complex. This investigation was conducted to study the status and integrity of major immune organs and tissues of broiler chickens at the early stage of respiratory disease complex by histopathology. Material and Method: This study included 20 flocks of broilers in initial stage of respiratory disease (disease group) at age of 25-30 and 5 flocks with no respiratory signs at the same age (negative control group). Initially history of each flock was taken and biosecurity status and management risk factors were assessed. Clinical evaluation and necropsy examination were performed. The gross lesions were scored and the size of bursa of Fabricius and spleen were evaluated by bursameter. Samples of trachea, bursa of Fabricius, thymus, spleen and cecal tonsils were collected and fixed in 10% neutral buffered formalin and were routinely processed for histological examination and scoring of the lesions. Results and conclusion: Based on gross lesion scores, trachea, air sacs and cecal-tonsil in disease group had more significant lesions than those in control group (p<0.05) but lung lesions did not show significant difference in two groups. Histopathological evaluation showed that the severity of lesions in trachea, bursa of fabricius, thymus, spleen and cecal-tonsil were significantly higher in disease group as compared with control group (P<0.05). It was noticed that the mean size of bursa of Fabricius and spleen in disease group was significantly smaller (P<0.05) than control group. According to the results, it was shown that biosecurity and management had no significant effects on estimated parameters. Collectively, the results obtained in this investigation indicate that early immunosuppression, most probably plays a relevant role in the outcome of respiratory disease complex and is a constant unseen element in etiopathogenesis of respiratory disease complex.

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