Page 357 - Veterinary Immunology, 10th Edition

these, the T cells interact with dendritic cells and passing B cells.
VetBooks.ir The B cell areas, in contrast, consist of round primary follicles

scattered through the sheaths. These follicles are sites where
germinal center formation, clonal expansion, isotype switching, and

somatic hypermutation occur.
The white pulp is separated from the red pulp by a marginal
sinus, a reticulum sheath, and a marginal zone of cells. This
marginal zone is an important transit area for white cells moving

between the blood and the white pulp. It is also rich in
macrophages, dendritic cells, and B cells. Most of the blood that
enters the spleen flows into the marginal sinus and through the
marginal zone before returning to the circulation through venous

sinuses. This flow pattern ensures that these antigen-presenting
cells can capture any blood-borne antigens and deliver them to the
B cells in the marginal zone. The white pulp is involved in adaptive
immune responses, whereas cells of the marginal zone can

participate in both innate and adaptive responses. White pulp does
not contain HEVs. Instead, lymphocytes enter the white pulp
through the marginal zone, although the route by which they leave
is unclear.

Function

Intravenously administered antigens are trapped in the spleen.
Depending on the species, they are taken up by dendritic cells in
the marginal zone or in the periarteriolar macrophage sheaths.
These dendritic cells and macrophages carry the antigen to the

primary follicles of the white pulp, from which, after a few days,
antibody-producing cells migrate. These antibody-producing cells
(plasma cells and plasmablasts) colonize the marginal zone and

move into the red pulp. Antibodies produced by these cells diffuse
rapidly into the bloodstream. Germinal center formation also occurs
in the primary follicles. In an animal possessing circulating
antibodies, trapping by dendritic cells within the follicles becomes
significant. As in a primary immune response, the antibody-

producing cells migrate from these follicles into the red pulp and
the marginal zone where antibody production occurs.

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