It is a specialized organ that, as first demonstrated by Bruce Glick and later by Max Dale Cooper and Robert Good, is necessary for B cell (part of the immune system) development in birds.
[2] In 1956, Bruce Glick showed that removal of the bursa in newly hatched chicks severely impaired the ability of the adult birds to produce antibodies.
This was a serendipitous discovery that came about when a fellow graduate, Timothy S. Chang, who was teaching a course on antibody production obtained chickens from Glick that had been bursectomised (removal of the bursa).
In thymectomized animals, the ability to reject allografts, and to mount delayed hypersensitivity responses, was drastically reduced.
Typically, the virus attacks the bursa of young birds, preventing development of the immune system.