anaphylactic shock UK /ˌænəfɪˈlæktɪk ˈʃɒk/US /ˌænəfəˈlæktɪk ˈʃɑːk/ noun1 •A severe and rapid systemic allergic reaction to an allergen, constricting the trachea and causing generalized vasodilation. formsanaphylactic shocks