Many a ground nest has been found by the unsuspecting lawn mower. If you do get stung, the natural tendency is to grab at the stinger and try to yank it out. When doing this you actually can squeeze more poison into your system. This can be avoided by using a fingernail or knife blade to scrape the stinger from the skin. Cleanse the wound and use an antiseptic, a cold compress should relieve pain and swelling.
Allergic reactions can range from slight to serious shock.
Any reaction more than local swelling and pain should be checked by a doctor. I consider myself lucky to have seen this amazing phenomenon twice. The first time was in my front yard where I had two fruit trees. The sound was what brought me outside to look. Then I saw them amongst the branches, moving en mass.