In a series of in vivo experiments, Nakai et al. (64) found that administration of B2-adrenergic receptor (B2AR) agonists resulted in a rapid reduction in the numbers of blood CD4+ and CD8+ T cells, with further investigations revealing this lymphopenia was the result of B2AR stimulation inhibiting lymphocyte egress from lymph nodes (64). This evidence concerns the gene ADRB2 and lymphopenia.