The separation of human and cellular immunity was further advanced by the study of immunodeficient humans and animals. For example, thymectomized or congenitally athymic animal as well as humans cannot carry out graft rejection, yet they are capable of producing some antibody responses. The reverse is also true. Children (and animals) who have an immune deficit in the humoral response do not make antibodies but can reject grafts and appear to handle viral, fungal, and some bacterial infections quite well. An extraordinary finding by Good and colleagues in studying the cloacal lymphoid organ in chickens revealed that, with the removal of the bursa Fabricius, these animals lost their ability to produce anti bodies and yet retained the ability to reject grafts.