Can antibodies be formed against white blood cells after blood transfusion?
Antibodies and other immune responses do indeed occur against the donor's white blood cells. However, white blood cells are present in relatively tiny quantities (3-10 million WBCs per ml of blood vs 4-6 billion cells per ml).

Because the number of WBCs is so low, the immune reaction to them is also proportionally lower, with fever being a typical (and minor) complication of the reaction to WBCs. Some blood banks remove WBCs from blood to prevent these reactions.

For more information on the possible types of reactions to blood transfusions, see Wikipedia.

EDIT: Even in cases where the transfusion does not produce any symptoms, an immune reaction is most assuredly happening. In those cases, the immune system is simply killing off the foreign WBCs without any fuss.