The bursa of Fabricius is the site of primary B cell generation in the
chicken. Analysis of the rate of bursal cell emigration demonstrated
that about 1% of the blood B cell pool was replaced per hour by bursal
emigrants. Surgical bursectomy distinguished three populations of blo
od B cells. About 60% of blood B cells in 3-wk-old chickens were short
lived, with a lifespan of 2 to 3 d, and included most bursal emigrant
s. These cells migrate directly from the bursal follicular cortex to t
he periphery, express the LT2 antigen, and are proposed to represent a
diversified repertoire of B cell specificities that have emigrated fr
om the bursa in the absence of interaction with environmentally derive
d antigens in the follicular medulla. About 35% of blood B cells were
much longer lived cells, having a lifespan exceeding 2 wk. These cells
represent about 10% of bursal emigrants, which do not express the LT2
antigen and do not divide in the peripheral blood following emigratio
n from the bursa. This population may represent cells that have underg
one positive selection in the bursa by interacting with environmentall
y derived antigens and should, therefore, contain a restricted reperto
ire of B cell specificities. The third population, about 5% of blood B
cells, were short-lived cells that represent the progeny of postbursa
l B cell production. The frequency of these cells progressively increa
ses with time, taking over from the short-lived bursal emigrants as th
e bursa involutes, likely as a diversified repertoire of B cell specif
icities.