noun, plural: agglutinins
Agglutinins in the blood plasma are responsible in making sure that there will be blood cells of only one blood type circulating in a person's body. If blood cells of a different blood type enter the body, agglutinins cause agglutination and destruction of these blood cells. For example people with blood type A have agglutinins b in their blood plasma. Hence, blood transfusion of blood type B will cause agglutinins b to react with the cells and cause agglutination.
Word origin: agglutin(ation) + -in