Pathology of lymphoid organs with emphasis on immunosuppression

A review of the literature is presented to include recent diagnostic and research histopathology accessions from poultry produced on the Delmarva peninsula region of the U.S.A. Emphasis is placed on immunosuppression (IS) and pathology of primary lymphoid organs of the chicken. The evolutionary relationship of the avian and mammalian lymphoid systems is based on a common reptilian past with 160 million years of evolutionary dichotomy. Four methods used to assess IS anatomically include: (1) recognition of marked reduction of lymphocytes in both primary and secondary lymphoid organs; (2) recognition of diseases commonly associated with IS; (3) recognition of known immunosuppressive diseases; (4) germinal center enumeration. An experimental method of qualifying and quantifying IS related to single or multiple insults is explored through the use of germinal center enumeration. The effect of stress and inanition on avian lymphocytes must be considered in histological evaluations as chickens are a steroid sensitive species.