ADRB2 and asthma: Sympathetic and parasympathetic neurons (and their reciprocal interactions with sensory neurons and immune cells) contribute to the clinical manifestations of asthma.46 Sympathetic neurons release the neurotransmitter noradrenaline, which induces airway smooth muscle relaxation via β2-adrenergic receptors (β2Ars) on smooth muscle cells.46 β2AR agonists, which are often used as bronchodilators for the treatment of asthma, may influence immune cells.