Page 351 - Veterinary Immunology, 10th Edition

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FIG. 12.15 The circulation of lymphocytes. T cells circulate in both
the bloodstream and the lymphatic fluid. Their precise route through
a lymph node depends on whether they are naïve or primed. Thus
naïve lymphocytes enter lymph nodes through the bloodstream and
the high endothelial venules. Primed lymphocytes, in contrast,
migrate through the tissues and enter through afferent lymphatics.
They all leave through efferent lymphatics.

Circulating T cells leave the bloodstream by two routes. T cells
that have not previously encountered antigens (“naïve” T cells)
bind to HEV in lymph nodes. The high endothelial cells in these
vessels are not joined by tight junctions but are linked by

discontinuous “spot-welded” junctions. This means that
lymphocytes can pass easily between the high endothelial cells.
Circulating lymphocytes can adhere to these high endothelial cells

and then migrate into the paracortex. The emigration of
lymphocytes from HEVs resembles that of neutrophils in inflamed
blood vessels. Thus, the cells first roll along the endothelial surface
binding to selectins. As they roll, they become activated and
express integrins. This results in their complete arrest and

emigration. The number and length of HEVs are variable and
controlled by local activity. Thus, stimulation of a lymph node by
the presence of antigens results in a rapid increase in the length of
its HEVs. If, however, a lymph node is protected from antigens, its

HEVs shorten. Recognizable HEVs are not normally found in
ruminant lymph nodes, but paracortical venules serve the same

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