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 development in birds. Mammals generally do not have an equivalent organ; the bone marrow is often both the site of hematopoesis and B cell development.

The ‘B’ in ‘B cell’ refers to bursa-derived. This is simply because during the 1960s B cells were first defined (and distinguished from thymus-derived T cells) in birds, which have a bursa. A decade later, after examining almost every other organ including the appendix, researchers finally discovered that mammalian B cells develop in the bone marrow. The fact that ‘bone marrow’, like bursa, starts with a ‘B’ is a coincidence.