What would cause a person to itch or get a rash while receiving a blood transfusion?

Allergic reaction: Abo and rh factors represent major differences between individuals' that are always typed before transfusion. There are many minor factors that can't be completely avoided. Sometimes people receiving blood with a minor factor difference have an allergic reaction i.E an itchy rash.

Typical Reaction: Just like dr. Machtinger stated, blood mismatches can cause itch and rash. But the simple infusion of a "foreign" blood product can stimulate your mast cells to release histamine causing the itch and rash. Unless something more serious happens, maybe they could slow down the infusion rate and/or pre-treat you with an antihistamine to prevent this reaction in the future.

Allergic reaction: There are two common types of transfusion side effects that can cause hives, rash or itching. A mild allergy to a component in the transfused blood or due to cytokines, which are chemicals that are normally present in donated blood, released from the white blood cells during collection and storage. Of course the rash or itch can sometimes be a sign of a more serious hemolytic type reaction.