The substance within the introduced cell which acts as a stimulus (antigen) to the body cells is called an agglutinogen.
agglutinogen ag·glu·tin·o·gen (āg'lu-tĭn'ə-jən, ə-glōōt'n-)
n.
An antigen that stimulates the production of a particular agglutinin, such as an antibody. Also called agglutogen.