The major crossmatch is designed to detect which of the following?

The major crossmatch is specifically designed to identify any recipient antibodies that might react with antigens present on the donor red blood cells. During this process, a sample of the recipient's serum is tested against the donor's red cells to ensure compatibility before a transfusion.

In instances where the recipient has existing antibodies against red cell antigens, a transfusion could lead to hemolytic reactions, which can be life-threatening. Therefore, the major crossmatch is crucial for ensuring that any such antibodies are detected, allowing for safe transfusion practices.

This process differentiates itself from other scenarios in the question. For example, mistyping a group A patient as group O primarily relates to pre-transfusion typing errors rather than the detection of specific antibodies. Similarly, while unexpected antibodies in a donor unit or mislabeling of donor units also pose risks, they do not specifically pertain to the primary objective of the major crossmatch, which is to prevent recipient antibodies from reacting adversely to the donor's red cells. The major crossmatch is an essential safety measure to ensure compatibility at this critical juncture of blood transfusion.