Foreman explains that the patient had antibodies for blood
type A and type B and that the blood banks test for antibodies and that this is why they wrongly deduced the patients' blood type. In fact the blood banks test for both antigens and antibodies, eliminating the possibility for this error. On top of that, a patient with antibodies for both blood types would be shown as having type O.
Setting off one sprinkler head does not trigger the entire system. Each head has an element that melts to apply water to just the areas that are on fire.
To check for DIC (blood type mismatch), House volunteers to transfuse some suspect blood because he's type AB. However, type AB is universal recipient, meaning that he won't have any DIC, no matter what type of blood is transfused into him.