Final answer:
The presence of an antibody to a low frequency antigen that the screening cells did not express can cause a positive crossmatch at AHG phase after a negative antibody screen.
Step-by-step explanation:
A common reason for a positive crossmatch at the anti-human globulin (AHG) phase after a negative antibody screen could be due to an antibody to a low frequency antigen. This type of antibody might not be detected in the initial antibody screen because the commercially prepared pooled type O red blood cells used in the screen may not express the rare antigen. However, these low-incidence antigens could still be present on the donor red blood cells, leading to a positive crossmatch when the patient's serum is mixed with the donor cells.