Page 349 - Veterinary Immunology, 10th Edition

thus are exposed to cytokines, such as IL-18, that are released when
VetBooks.ir the macrophages encounter invaders. The innate lymphocytes in

turn rapidly increase IFN-γ secretion that activates the
macrophages still further and enhances their antimicrobial

activities.
An interesting feature of secondary lymphoid organs is the fact
that both B and T cells are highly active and motile. The T cells in
the paracortex and B cells in the cortex are guided by follicular

dendritic cells. Chemokines control the relocation and recirculation
of lymphocytes and ensure that they end up in the right place. For
example, T cells are attracted to the perifollicular area of the cortex.
B cells, on the other hand, are attracted to the interior of germinal

centers. When T cells are activated, they too may enter germinal
centers where they “help” B cells respond to antigens. Other
secondary lymphoid organs employ different homing receptors. For
example, MAdCAM-1 is a homing receptor found in blood vessels

in PPs. Lymphocytes that recirculate to the intestine express high
levels of the MAdCAM-1 ligand.
Soluble antigens entering the node through its afferent
lymphatics first pass into the subcapsular sinus. From there they

enter the conduit network and are carried into the cortex. Larger
antigens such as viruses are captured by macrophages within the
subcapsular sinus. These macrophages carry the viral particles
through the sinus floor and directly present them to B cells in the

underlying follicles. The B cells then migrate to the T cell–B cell
boundary, where they receive specific T cell help. B cells can also
enter the paracortex directly from HEVs. There is a specialized
population of follicular dendritic cells clustered around these blood

vessels so that immigrating B cells can survey any antigens they
may be carrying. This is also a perfect location to receive T cell help.
When bacteria invade tissues, the resident dendritic cells are
activated and migrate to the draining lymph node, where they

accumulate in the paracortex and cortex. These dendritic cells form
a web through which antigens must pass. Captured antigens are
presented by the dendritic cells to T cells. T cells are initially
activated in the paracortex, whereas the B cells remain randomly
dispersed in the primary follicles. Both cell populations then

migrate to the edges of the follicles where they interact. Once

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