Page 744 - Veterinary Immunology, 10th Edition

placenta from mother to fetus; are largely prevented from crossing
VetBooks.ir totally prevented from crossing in an epitheliochorial species such
in an endotheliochorial species such as dogs and cats; and are

as horses or pigs.

Dogs and cats have an endotheliochorial placenta in which the
chorionic epithelium is in contact with the endothelium of the
maternal capillaries. In these species, 5% to 10% of IgG is directly

transferred from the mother to the puppy or kitten, but most must
be obtained through colostrum. (It is interesting to note that
elephants also have an endotheliochorial placentation and there is

significant prenatal passive transfer of immunity in this species.)
The placenta of ruminants is syndesmochorial; that is, the
chorionic epithelium is in direct contact with uterine tissues,
whereas the placenta of horses and pigs is epitheliochorial and the
fetal chorionic epithelium is in contact with intact uterine

epithelium. In mammals with both these types of placenta, the
transplacental passage of immunoglobulin molecules is totally
prevented. Thus their newborns are entirely dependent on

antibodies received through the colostrum.

Secretion and Composition of Colostrum

and Milk

Colostrum contains the accumulated secretions of the mammary

gland over the last few weeks of pregnancy together with proteins
actively transferred from the bloodstream under the influence of
estrogens and progesterone. Neonatal immunoglobulin receptors
(FcRn) are expressed in mammary gland ductal and acinar cells and

mediate the transfer of IgG from serum into colostrum. Colostrum
is therefore rich in IgG and IgA and contains some IgM and IgE
(Table 23.1). The predominant immunoglobulin in the colostrum of
most domestic mammals is IgG, which may account for 65% to 90%

of its total antibody content; IgA and the other immunoglobulins
are usually minor but significant components. As lactation
progresses and colostrum changes to milk, differences among
species emerge. In primates, IgA predominates in both colostrum
and milk. In pigs and horses, IgG predominates in colostrum, but

its concentration drops rapidly as lactation proceeds so that IgA

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