The present invention provides a CTLA-4 non-blocking agent of a complete human antibody nature, thus is non-immunogenic in a human. The immunoassay method using such a non-blocking agent measures the CTLA-4 content in a sample of a human subject. The present invention further provides a novel method for isolating human regulatory T cells. The resultant enriched and depleted cellular populations are useful in treating or ameliorating of human diseases.