The CD28/T-cell receptor (TCR)/CTLA4 complex regulates T-cell homeostasis and tolerance in HT, while CD45 protein tyrosine phosphatase (PTPase) cooperates with the vitamin D receptor to interact with the TCR complex and influence the Th1/Th17/Treg pathways that are critical to the development of HT. Here, CTLA4 is linked to hematocrit.