Page 727 - Veterinary Immunology, 10th Edition

VetBooks.ir Development of the Immune System

The development of the immune system in the mammalian fetus
follows a consistent pattern. The thymus is the first lymphoid organ

to develop, followed by the secondary lymphoid organs. B cells
appear soon after the development of the spleen and lymph nodes,
but antibodies are not usually detectable until late in fetal life. The
ability of the fetus to respond to antigens develops very rapidly
once the lymphoid organs appear, but all antigens are not equally

capable of stimulating fetal lymphoid tissue. The immune system
develops in a series of steps, with each step permitting the fetus to
respond to more antigens. These steps are driven by a gradual

increase in the use of gene conversion or somatic mutation to
increase antibody diversity. The ability to mount cell-mediated
immune responses develops at the same time as antibody
production. T cell receptor (TCR) diversity is also limited in the
fetus and neonate, and their cytokine production may be low. This

may simply be due to their lack of exposure to foreign or microbial
antigens.

Specific Animal Immune Systems

Foal

The gestation period of the mare is about 340 days. Lymphocytes
are seen first in the thymus at about 60 to 80 days post-conception.
They are found in the mesenteric lymph node and intestinal lamina

propria at 90 days and in the spleen at 175 days. Blood lymphocytes
appear at about 120 days. A few plasma cells may be seen at 240
days. Graft-versus-host disease, a cell-mediated response, has
developed in immunodeficient foals transplanted with tissues from
a 79-day-old fetus. The equine fetus can respond to coliphage T2 at

200 days post-conception and to Venezuelan equine encephalitis
virus at 230 days. VDJ sequence diversity increases as the fetus
develops and as the foal develops into an adult. Newborn foals
have detectable quantities of IgM and IgG and occasionally IgG3 in

their serum, but IgE production in the horse does not begin until
foals are 9 to 11 months of age. Like other large herbivores, the foal

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