The present invention relates to assay methods used for detecting the presence of PIF, and to PIF peptides identified using this assay. In particular, the present invention relates to flow cytometry assays for detecting PIF. It is based, at least in part, on the observation that flow cytometry using fluorescently labeled anti-lymphocyte and anti-platelet antibodies demonstrated an increase in rosette formation in the presence of PIF. It is further based on the observation that flow cytometry demonstrated that monoclonal antibody binding to CD2 decreased in the presence of PIF. The present invention further relates to PIF peptides which, when added to Jurkat cell cultures, have been observed to either (i) decrease binding of anti-CD2 antibody to Jurkat cells; (ii) increase expression of CD2 in Jurkat cells; or (iii) decrease Jurkat cell viability. In additional embodiments, the present invention provides for ELISA assays which detect PIF by determining the effect of a test sample on the binding of anti-CD2 antibody to a CD2 substrate.