hard to know but your adaptive immune system is built around a very large group of proteins called the Major Histocompatibility Comples or the MHCs. These proteins are what control whether or not T-cells become activated. They therefore control activation of cytotoxic T-cells to kill viral infected cells and control T helper cells to make B cells make antibodies. The MHCs also are unique to you and you alone. MHCs are made from genes called the Human Leukocytic Antigens or HLAs. We know that certain combinations of the HLA genes are linked to particular diseases. Maybe a person susceptibility or lack of susceptibility is due to a particular set of HLAs.