Distinct HLA‐DR epitopes and distinct families of HLA‐DR molecules defined by 15 monoclonal antibodies (mAb) either anti‐DR or allo‐anti‐Iak crossreacting with human DR molecule. I. Cross‐inhibition studies of mAb cell surface fixation and differential binding of mAb to detergent‐solubilized HLA molecules immobilized to a solid phase by a first mAb