I am allergic to milk casein, can I have whey?

Probably not... Cow's milk contains casein (alphas1-, alphas2-, beta-, and kappa-caseins) & whey (alpha-lactalbumin, ala; beta-lactoglobulin, blg; bovine lactoferrin; bovine serum albumin, bsa; and bovine immunoglobulins). Acute allergic responses to milk are due to ige directed against various allergens in milk. All milk proteins are potential allergens & polysensitization to several proteins occurs in most pts.
In theory, yes. If you have anaphylactic allergy to casein i'd steer clear of whey because you can't be certain that it's 100% free of casein. And if some whey products are, others may not be. So its russian roulette. Incidentally, tuna fish packed in water is routinely packed in a little cow's milk, watch out. Tuna packed in oil is safe. 92% of cow' milk allergic persons are also allergic to goat's milk.