Hornets have stingers used to kill prey and defend nests. Hornet stings are more painful to humans than typical wasp stings because hornet venom contains a large amount (5%) of acetylcholine. Individual hornets can sting repeatedly; unlike honey bees, hornets do not die after stinging because their stingers are very finely barbed (only visible under high magnification) and can easily be withdrawn, so are not pulled out of their bodies when disengaging.
The toxicity of hornet stings varies according to hornet species; some deliver just a typical insect sting, while others are among the most venomous known insects. Single hornet stings are not in themselves fatal, except sometimes to allergic victims. Multiple stings by hornets (other than V. crabro) may be fatal because of highly toxic species-specific components of their venom.
The stings of the Asian giant hornet (V. mandarinia) are among the most venomous known, and are thought to cause 30–50 human deaths annually in Japan. Between July and September 2013, hornet stings caused the death of 42 people in China. Asian giant hornet's venom can cause allergic reactions and multiple organ failure leading to death, though dialysis can be used to remove the toxins from the bloodstream.
People who are allergic to wasp venom may also be allergic to hornet stings. Allergic reactions are commonly treated with epinephrine (adrenaline) injection using a device such as an epinephrine autoinjector, with prompt follow-up treatment in a hospital. In severe cases, allergic individuals may go into anaphylactic shock and die unless treated promptly. In general, Vespa stings induce the release of histamine due to the various mastoparans that they contain. However V. orientalis mastoparan is the interesting exception because it does not induce histamine increase in victim tissue - because it does not cause mast cell degranulation - and is not immunogenic.
Read more, here.