Lymphoid system

The primary lymphoid organs of birds are the thymus, in which the differentiation of T lymphocytes takes place, and the Bursa of Fabricius, a primary lymphoid organ unique to birds in which B lymphocytes develop. From these organs, lymphocytes migrate to secondary lymphoid organs. These secondary lymphoid organs include the spleen, mucosa –associated lymphoid tissue (BALT and GALT), lymphoid nodules in other organs and, in some aquatic birds only, lymph nodes. The Bursa of Fabricius can also act as a secondary lymphoid organ.