Serum is use to suppress Blade's thirst. Because of his vampiric nature, Blade has a thirst means he needs blood to process regularly. After Whistler adopted him he created the serum. It is a substitute for blood, but it is only one fourth as strong as blood. The only catch was that he needed it injected or orally taken, but it was not as good as blood. Blade on many occasions have proven to be unbelievably more powerful while he consumed blood instead of the serum.