Allergenicity of Milk Proteins
The proportion of the population with allergies caused by food has increased recently in developed nations. It is unfortunate, indeed, when the food that we eat makes us ill, because food is indispensable for life. The Ministry for Health and Welfare of J
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KAMINOGAWA AND
13.1
M.
TOTSUKA
INTRODUCTION
The proportion of the population with allergies caused by food has increased recently in developed nations. It is unfortunate, indeed, when the food that we eat makes us ill, because food is indispensable for life. The Ministry for Health and Welfare of Japan reported that 12.6% of Japanese children aged six years or less have food allergies (Iikura et at., 1999). There are many kinds of food allergens, the substances that cause food allergies. Cow's milk protein is one of the most common food allergens in many developed countries (Eigenman and Sampson, 1997; Chiaramonte et at., 1999). In this chapter, we describe the characteristics of allergy to cow's milk and its mechanism of onset, and discuss the allergenicity, the ability to trigger an allergic reaction, of various cow's milk proteins and the sites of the molecules recognized by the immune system. We also describe the results of recent studies on the suppression of allergic reactions by introducing changes in the antigenic sites of an allergen through amino acid substitution.
13.2
CHARACTERISTICS OF MILK ALLERGY
The onset of cow's milk allergy is generally during infancy. Varying estimates of the incidence from 1.8 to 7.5% of infants have been reported (H0st, 1997). The variation seems to be attributable to differences in diagnostic criteria, study design and the population being studied. The incidence in developed countries with strict diagnostic criteria can be estimated as
Advanced Dait·\· Chemistrl' Volume 1. Proteins, 3rd edn. Edited by P.F. Fox and P.L.H. McSweeney. Kluwer Academic/Plenum Publishers, 2003.
648
ALLERGENICITY OF MILK PROTEINS
2-5% (for reviews see Bahna and Heiner, 1980; Savilahti, 1981; Savilahti and Kuitunen, 1992; Businco and Bellanti, 1993; Hest, 1994, 1997; Hest et al., 1995; Wal, 1998).
One of the reasons for the increased proportion of infants with allergy to cow's milk seems to be the decrease in the number of mothers who breastfeed their infants for a prolonged period (Savilahti, 1981). The safest food for infants, with few exceptions, is breast milk. Cow's milk formulae which have components similar in kind and proportion to those of breast milk, are the usual replacement for breast milk. Studies on prevalence have shown that 0.5% of exclusively breast-fed infants, 7% of bottle-fed babies and 1.5 to 2.0% of all children have allergy to milk (Bahna and Heiner, 1980). During infancy, a small part of milk proteins that are allergenic may be absorbed through the intestinal wall without being digested because the intestinal tract is immature. This immaturity is considered to be one important cause of the onset of food allergy in infancy, although the degree of permeability of the intestine is not directly related to the risk of developing food allergy in general. The other main factor seems to be the insufficient functioning of the immune system of infants. A fully functioning immune system would suppress the onset of food allergies (see Section 13.3 below). Atopic dermatitis
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