'B' stands for bursa or its equivalent, bone.
Potterat
J J
JJ
D000818:Animals; D001402:B-Lymphocytes; D001854:Bone Marrow Cells; D002060:Bursa of Fabricius / Q000166:cytology*; D002645:Chickens / Q000033:anatomy & histology*; D006801:Humans; D009626:Terminology as Topic