Both in humans and in experimental animals, much of the airway hyperresponsiveness that accompanies viral infections is the result of increased reflex bronchoconstriction. The M3 muscarinic receptors on the airway smooth muscle function normally during viral infections so that the direct effects of acetylcholine on the smooth muscle are not altered. In contrast, the M2 muscarinic receptors on the vagal nerve endings, which normally inhibit acetycholine release, are markedly dysfunctional during viral infections. This leads to substantial increases in acetylcholine release and potentiated reflex bronchoconstriction. Multiple mechanisms account for virus-induced M2 receptor dysfunction. Viral neuraminidase may deglycosylate the M2 receptor, decreasing acetylcholine affinity. Furthermore, both viruses and interferon-γ decrease M2 receptor gene expression. Finally, in atopic hosts, viral infection causes M2 receptor dysfunction by activating eosinophils, causing them to release major basic protein which binds to the M2 receptor, functioning as an endogenous antagonist.