Final answer:
In transfusion testing, antigen-antibody reactions are observed by agglutination, which is the clumping together of cells or particles due to the binding of antibodies to antigens.
Step-by-step explanation:
In transfusion testing, antigen-antibody reactions are observed by agglutination. Agglutination is the process of clumping together of cells or particles due to the binding of antibodies to antigens. It is commonly used in blood typing and crossmatching to determine blood compatibility for transfusions.