What is a major source of background in the flow crossmatch?

In the context of flow crossmatching, the major source of background is due to the presence of Fc receptors, specifically FcR, on various immune cells. These receptors can bind to the Fc region of antibodies, leading to a significant increase in nonspecific binding that complicates the results of the crossmatching test.

This nonspecific binding often results in higher background noise in flow cytometry assays, which can mask the specific binding events that are important for accurate assessment of compatibility between donor and recipient tissues. When assessing the binding of donor-specific antibodies to cells, FcR-mediated interactions can contribute to false-positive or misleading signals, making it challenging to interpret the results correctly.

The other options, while important components within the immune system, do not contribute as significantly to background noise in flow crossmatches. For instance, CD4 and CD8 are primarily markers for T-cell subsets and are involved in adaptive immune responses, while HLA class II molecules are vital for antigen presentation to CD4+ T cells. However, they do not play a major role in generating background interference during crossmatching assays. Thus, the involvement of Fc receptors significantly impacts the background and must be accounted for when analyzing flow crossmatch results.