What is "bursa of fabricius"

In birds, the bursa of Fabricius ( Latin: Bursa cloacalis or Bursa fabricii) is the site of hematopoiesis, a specialized organ that, as first demonstrated by Bruce Glick and later by Max Cooper and Robert Good, is necessary for B cell (part of the immune system) development in birds. Mammals generally do not have an equivalent organ; the bone marrow is often both the site of hematopoiesis and B cell development. The bursa is present in the cloaca of birds and is named after Hieronymus Fabricius who described it in 1621.