Page 330 - Veterinary Immunology, 10th Edition

VetBooks.ir Primary Lymphoid Organs

The organs that regulate the development of lymphocytes are called
primary lymphoid organs. Lymphocytes fall into two major

populations called T cells and B cells, based on the primary organ in
which they mature. Thus all T cells mature in the thymus. B cells, in
contrast, mature within different organs depending on species.
These include the bursa of Fabricius in birds, the bone marrow in
primates and rodents, and the intestinal lymphoid tissues in rabbits

and ruminants. These primary lymphoid organs all develop early in
fetal life. As animals develop, newly produced, immature
lymphocytes migrate from the bone marrow to the primary

lymphoid organs, where they mature (Table 12.1). The primary
lymphoid organs are not sites where lymphocytes encounter
foreign antigens, and they do not enlarge in response to antigenic
stimulation.

TABLE 12.1

Comparison of Primary and Secondary Lymphoid Organs

Primary Secondary
Origin Ectoendodermal junction or endoderm Mesoderm
Time of development Early in embryonic life Late in fetal life
Persistence Involutes after puberty Persists in adults
Effect of removal Loss of lymphocytes No or minor effects
Response to antigen Unresponsive Fully reactive
Examples Thymus, bursa, some Peyer's patches Spleen, lymph nodes

Thymus

The thymus is located in the thoracic cavity in front of and below
the heart. In horses, cattle, sheep, pigs, and chickens, it also extends

up the neck as far as the thyroid gland. The size of the thymus
varies, its relative size being greatest in the newborn animal and its
absolute size being greatest before puberty. It may be very small
and difficult to find in adult animals.

Structure

The thymus consists of lobules of loosely packed epithelial cells,

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