Page 352 - Veterinary Immunology, 10th Edition

function.
VetBooks.ir through conventional blood vessels in tissues and are then carried
In contrast to naïve T cells, memory T cells leave the bloodstream

to lymph nodes through afferent lymphatics. They leave the lymph

nodes through the efferent lymphatics. Typically, afferent lymph in
sheep contains 85% T cells, 5% B cells, and 10% dendritic cells.
Efferent lymph contains greater than 98% lymphocytes, of which
75% are T cells and 25% are B cells. The efferent lymphatics

eventually join together to form large lymph vessels. The largest of
these lymph vessels is the thoracic duct, which drains the lymph
from the lower body and intestine and empties it into the anterior
vena cava.

Species Differences

Domestic pigs and related swine, hippopotamuses, rhinoceroses,
and some dolphins are different. Their lymph nodes consist of
several lymphoid “nodules” oriented so that the cortex of each
nodule is located toward the center of the node, whereas the

medulla is at the periphery (Fig. 12.16). Each nodule is served by a
single afferent lymphatic that enters the central cortex as a lymph
sinus. Thus afferent lymph is carried deep into the node. A cortex

surrounds the lymph sinus. Outside this region are a paracortex
and a medulla. This medulla may be shared by adjacent nodules
(Fig. 12.17). Lymph passes from the cortex at the center of the node
to the medulla at the periphery before leaving through the efferent
vessels that drain the region between nodules. The cortex and

paracortex have a similar structure to that seen in other mammals.
The medulla has very few sinuses but consists of a dense mass of
cells that is relatively impermeable to cells in the lymph. As a result,

few cells migrate through the medulla. T cells in these species enter
the lymph node in the conventional way through HEVs. However,
they do not leave the lymph node through the lymphatics but
migrate directly back to the bloodstream through the HEVs of the
paracortex (Fig. 12.18). Very few lymphocytes are found in pig

lymph.

352