Cows' milk is an important component of the diet especially during infancy. Yet, cows' milk can elicit allergic and other sensitivity reactions in some individuals. Cows' milk allergy (CMA) results from an abnormal immunologic reaction to cows' milk proteins. IgE responses are definitely involved in CMA. Immune complexes and tissue lymphocytes may also play a role in some forms of CMA, but further evidence is needed to firmly establish this possibility. The presence of circulating antibodies to cows' milk proteins of the IgG, IgA and IgM classes is not clinically significant. Such antibodies are found in both normal and allergic individuals. (β-Lactoglobulin and casein are the most common cows' milk allergens, although other cows' milk proteins may play important roles in some cases. Partial digestion of cows' milk proteins may enhance their allergenicity, whereas complete hydrolysis abolishes their allergenicity. Heating can also alter the allergenicity of the cows' milk proteins, but rather severe heating is required. More research will be necessary to develop hypoallergenic processing methods for cows' milk.