There are 12 allergens associated with cows, 9 of them relate to an allergy to the milk they produce (rather than the consumption of meat they produce).
The allergenic proteins in cow's milk are generally split into albumins (commonly called whey) and casein. In human milk the split is usually 60% whey to 40% casein. In cow's milk the split is 20% whey to 80% casein. This massive difference in composition is thought to be the cause of what causes IgE allergic reaction to cow's milk, with most people being allergic to casein proteins in the milk.
Cow's Milk Protein Allergy (also called CMPA) is not be confused with lactose intolerance. A true allergy is to proteins in a food, lactose intolerance is due to a reaction to sugars in milk. Lactose free products should not be used by people with an allergy as they may still contain proteins - always check your labels!
Cow's milk allergy is not currently associated with any syndromes.
There is a lot of evidence of cross reactivity between cow's milk and eggs and this is due to serum albumin proteins found in both.
About 13 - 20% of cow's milk allergy sufferers will also react to beef when consumed. This is, again, due to serum albumin proteins found in both foods.
Cow's milk allergy has also been linked to a soya allergy - the soya protein component that cross-reacts with casein has been identified as a glycinin molecule.