Page 350 - Veterinary Immunology, 10th Edition

antibody production is stimulated, the progeny of these B cells
VetBooks.ir move to the medulla and begin to secrete antibodies. Some of these

antibody-producing cells may escape into the efferent lymph and
colonize downstream lymph nodes. Several days after antibody

production is first observed in the medulla, germinal centers appear
in the cortex.
Some T cell–dendritic cell interactions are long-lasting, and in the
presence of antigen, T cells and dendritic cells form stable

complexes for many hours. However, before selecting its partner, a
dendritic cell might sample as many as 500 different T cells/hour
and can interact with up to 10 simultaneously (see Fig. 10.6).
Adherence to follicular dendritic cells is the predominant means

of antigen trapping once an animal has been sensitized by previous
exposure to an antigen. In a secondary response the germinal
centers become less obvious as activated memory cells emigrate in
the efferent lymph. Once this stage is completed the germinal

centers redevelop.
Antigen-stimulated lymph nodes also trap lymphocytes.
Interactions between infectious agents and mast cells result in the
production of tumor necrosis factor-α (TNF-α). The TNF-α blocks

the passage of lymphocytes through these organs, the lymphocytes
accumulate, and the lymph nodes swell. This trapping concentrates
lymphocytes close to sites of antigen accumulation. After about 24
hours the lymph nodes release their trapped cells and their cellular

output is increased for several days.

Lymphocyte Circulation

In adult animals, most cell types reside in stable tissues and do not
move a lot. The many different cells of the immune system are, in

contrast, highly mobile. Cells move from the bone marrow to the
thymus and secondary lymphoid organs; cells migrate around the
body looking for invaders and also move from lymphoid organs to
sites of microbial invasion. T cells, for example, constantly circulate
around the body in the blood and tissue fluid and are the

predominant lymphocytes in blood (Fig. 12.15). As they travel, they
survey the body for foreign antigens and preferentially home to
sites of microbial invasion and inflammation.

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