Although acute sympathetic activation, such as that observed during exercise, may be beneficial for improving natural killer cell mobilisation and function (Schedlowski 1993; Idorn 2016; Pedersen 2016), early studies assessing the impact of repeated activation of β2-adrenergic receptors (β2-AR) on natural killer cells found it to be immunosuppressive and to lead to progression of a natural killer cell-sensitive tumour model (Shakhar 1998). This evidence concerns the gene ADRB2 and neoplasm.