Dairy Chemistry And Biochemistry

196 DAIRY CHEMISTRY AND BIOCHEMISTRY

41

0

141 A

Net charge

-10

-8

0

Figure 4.27 Model of the bovine serum albumin molecule.

Igs are very complex proteins which will not be reviewed here. Essential-
ly, there are five classes of Ig: IgA, IgG, IgD, IgE and IgM. IgA, IgG and
IgM are present in milk. These occur as subclasses, e.g. IgG occurs as IgG,
and IgG,. IgG consists of two long (heavy) and two shorter (light)
polypeptide chains linked by disulphides (Figure 4.28). IgA consists of two
such units (i.e. eight chains) linked together by secretory component (SC)
and a junction (J) component, while IgM consists of five linked four-chain
units (Figure 4.29). The heavy and light chains are specific to each type of
Ig. For a review of immunoglobuins in milk, see Larson (1992).
The physiological function of Ig is to provide various types of immunity
in the body. The principal Ig in bovine milk is IgG, while in human milk it
is IgA. The calf (and the young of other ruminants) is born without Ig in its
blood serum and hence is very susceptible to infection. However, the
intestine of the calf is permeable to large molecules for about 3 days
postpartum and therefore Ig is absorbed intact and active from its mother’s
milk; Igs from colostrum appear in the calves blood within about 3 h of