What adverse effect can excess antibodies cause during a crossmatch assay?

In a crossmatch assay, excess antibodies can lead to false positive reactions. This occurs because a higher concentration of antibodies may bind non-specifically to antigens on the cells being tested or to other proteins within the assay system. As a result, the assay may mistakenly indicate a match where there isn't one, thereby compromising the accuracy of the test.

While other factors such as enhancement of cell lysis, increased binding of complement, and high background noise can occur in immunological assays, they are less directly related to the phenomenon of false positives as caused by excess antibodies in the context of crossmatching. False positives are particularly problematic because they can lead to incorrect conclusions about compatibility, influencing clinical decisions related to transplantation.