Why can't scientists inject horse blood into humans?

So as most of you know, scientists inject germs into horses, since horse blood can easily create a antibody which can then be used to create a type of vaccine.

However, instead of injecting the disease into the horse blood, why can't horse blood be injected into humans? The immune system of the horse can create an antibody for every disease in our body as soon as we get it?

The diseases that horses get can be negated by the horse blood as well, since it is already used to diseases that horses can get.