Researchers at the Institute of Anatomy at University Hospital Bonn administered cow's milk constituent casein together with an effect enhancer to mice, which went on to develop neurological disorders. Electron microscopy showed the myelin, the insulating layer around the nerve fibers, was massively perforated in the mice – apparently triggered by casein administration. 

But to what extent can the results be transferred to people with MS? To answer this question, the researchers added casein antibodies from mice to human brain tissue. The researchers found they did indeed accumulate in the cells responsible for myelin production in the brain.

Certain white blood cells, B cells, are responsible for antibody production. The study found that B cells in the blood of people with MS respond particularly strongly to casein. Presumably, the affected individuals developed an allergy to casein at some point as a result of consuming milk. Now, as soon as they consume fresh dairy products, the immune system produces masses of casein antibodies. Because of cross-reactivity with a protein called MAG, these also damage the myelin sheath around the nerve fibers. Such cross-reactivity can occur when two molecules are very similar, at least in parts. The immune system then in a sense mistakes them for each other. This means that in the casein-treated mice, the body's own defenses were also directed against MAG, destabilizing the myelin.

However, this only affects MS patients who are allergic to cow's milk casein. It is possible that cow's milk also increases the risk of developing MS in healthy individuals. Once such an immune response exists, cross-reactivity with myelin can in theory occur. However, this does not mean that hypersensitivity to casein necessarily leads to the development of MS, the researchers emphasized. This would presumably require other risk factors.

The findings were published in the journal Proceedings of the National Academy of Sciences.