196 DAIRY CHEMISTRY AND BIOCHEMISTRY 1 A Net charge 141A Figure 4.27 Model of the bovine serum albumin molecule Igs are very complex protei ich will not be reviewed here. Essential- ly, there are five classes of Ig IgG, IgD, IgE and IgM. IgA, IgG and gM 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 mothers milk; Igs from colostrum appear in the calves blood within about 3 h of196 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