Antigen: A substance that when introduced into the body stimulates the production of an antibody. Antigens include toxins, bacteria, foreign blood cells, and the cells of transplanted organs.
Complement protein: Substance that is produced by a predecessor protein or in response to the presence of foreign material in the body and that triggers or participates in a complement reaction.
Dilation (Vasodilation) : The abnormal enlargement or widening of the lumen of blood vessels in response to a variety of stimuli.