Milk proteins comprise two distinct classes of protein, in a 4-1 ratio of caseins to non-caseins. Micellar casein is a slow release protein rich in branched chain amino acids, which aid in the building of new muscle mass, and is popular with athletes and bodybuilders in preventing the overnight breakdown of muscle tissue.
In addition to micellar casein, milk contains dozens of other more water-soluble proteins than the caseins, which do not form clots of curds. These tend to be known generally as whey proteins, because they are what are left behind after the clotting of the casein proteins.
Milk consists of roughly 20% whey proteins, with lactoglobulin making up about 50% of that amount, another 20% being lactalbumin, and the rest made up of many minor proteins and enzymes. Some other whey proteins include serum albumin and immunoglobulins (antibodies), which are critical in the immune system for identifying and neutralising threats like viruses and bacteria. Immunoglobulin levels are highest in milk during the colostrum, or first milk phase of late pregnancy in lactating mammals.
Milk protein is also rich in glutamine, which can help to support the immune system, prevent muscle wasting and combat bacterial infection in the gut.