What characterizes a positive T cell crossmatch by flow cytometry?

A positive T cell crossmatch by flow cytometry is characterized by the presence of IgG antibodies. This is significant because IgG antibodies are capable of mediating complement-dependent cytotoxicity and can activate the complement system, which can lead to the destruction of the target cells. In the context of transplantation, the presence of IgG against donor-specific HLA antigens indicates that the recipient has pre-existing antibodies that could cause graft rejection.

In flow cytometry, the interaction between T cells and these IgG antibodies can be measured, allowing for the identification of potential incompatibilities before a transplant.

On the other hand, IgM antibodies and low titer responses are typically not indicative of a significant risk for transplant rejection in the context of T cell crossmatching, as IgM is usually less harmful and low titers suggest a minimal reaction. Additionally, non-HLA antibody targets do not represent the same risk for graft rejection as HLA-specific IgG antibodies do, thus they are not a focus in T cell crossmatch assessments.