Type O negative blood has no antigens (and is the only blood type to have none), so it won't trigger an immune response, even if the recipient has a different blood type. Antigens are surface proteins found on red blood cells. Blood type A has an A antigen, while blood type B has a B antigen. Blood type AB has both A and B antigens. Blood type O has neither.
Additionally, blood is classified based on the presence of the Rh factor. Blood types with the Rh factor are considered Rh positive, and those without it are considered Rh negative. When we account for A and B antigens as well as the Rh factor, we get the eight most common blood types: A+, A-, B+, B-, O+, O-, AB+ and AB-.
Our immune system protects us from invaders like bacteria and viruses. It can recognize the antigens present in our own blood type but may go into defense mode if it detects antigens from a different blood type. If mismatched blood is given during a transfusion, the body may fight back, potentially endangering the life of the patient.
It’s also important to know that, unlike the other blood types, patients with type O negative blood can only receive their same blood type. This also increases the need for O negative blood donors; having an ample supply helps ensure that there’s enough blood for all types, including patients with O negative blood.