What happens when incompatible blood is mixed and what is the main cause of this phenomenon?

Agglutination of erythrocytes occurs, in other words, adhesion. The process is based on the presence of certain antibodies in the blood plasma and antigens on erythrocytes. If these sets are compatible, fit like a key to a lock, no reaction develops. When transfused with an incompatible blood group, with incompatible sets, an immune response is triggered, aimed at destroying foreign red blood cells. The whole process is called blood transfusion shock.