Final answer:
Mixing blood type A with anti-A serum leads to agglutination, where the RBCs clump together, potentially causing dangerous blockages. It's vital to match donor and recipient blood types in transfusions to avoid such reactions.
Step-by-step explanation:
If blood type A was mixed with anti-A serum, the anti-A antibodies present in the serum would bind to the A antigens on the red blood cells (RBCs). This would result in agglutination, where the RBCs clump together, and could potentially lead to serious complications such as blockage of blood vessels. This is an immune response where the body recognizes the A antigens as foreign and the anti-A antibodies target them for destruction.
Individuals with blood type A naturally have anti-B antibodies, not anti-A, as these would attack their own red blood cells. For this reason, it is crucial during blood transfusions to match the blood type of donor and recipient to prevent such dangerous reactions.