Page 731 - Veterinary Immunology, 10th Edition

initially use all its IGHV or IGHD genes. Likewise, N-region
VetBooks.ir addition does not occur before day 40, suggesting that the onset of

terminal deoxynucleotidyltransferase activity occurs after that time.
IgM, IgA, and IgG transcripts are present from 50 days in all major

lymphoid organs. Piglets are thus born with relatively limited B cell
diversity. B cell numbers increase for the first 4 weeks after birth,
but their antigen-binding repertoire does not begin to expand until
4 to 6 weeks of age.

The production of IgA is controlled by exposure to the intestinal
microbiota (Box 23.1). A limited amount of B cell class switching
occurs in the developing fetus so that, as a result, newborn piglets
already possess some intestinal IgA. Thus piglets at birth are

exposed to new antigens from the growing microbiota as well as
from colostrum and milk. It is these antigens that initiate IgA
production. At weaning, the animal is exposed to new dietary
antigens and must also develop oral tolerance.

Box 23.1

The Inheritance of Fecal IgA Levels

In inbred mice, fecal IgA levels are highly variable. Some have high

—while others have low—fecal IgA. If the microbiota of low-fecal
IgA mice are transferred to high-fecal IgA mice, the recipients' fecal
IgA levels promptly drop. So the quantity of IgA in the feces is
determined by the microbiota. The bacteria from IgA-low mice
appear to be able to degrade both secretory component and IgA. It

is generally recognized that a young animal receives its
microbiome from its mother. As a result, fecal IgA levels are
vertically transmitted from the mother and just appear to be

inherited. If this process occurs in domestic mammals, it may
explain many of the enteric disease problems associated with
intensive production of pigs and other species.

Lambda chain rearrangements can be detected in the developing
yolk sac of piglets between 20 and 50 days' gestation well before
kappa rearrangements. Junctional diversity within VDJ regions is

limited at all stages of development. B cell lymphogenesis and gene
rearrangements continue for at least 5 weeks postpartum. All of

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