It’s almost that time of year again – your flu shot is looming. It “primes” your immune system cells so that they will activate if and when they encounter that foreign substance again. Whenever a foreign “threat” shows up, your immune system also releases mediators like histamine, which can cause inflammation around an area of concern. If your body responds to the shot with histamine, the site of the shot – usually your upper arm – could be sore for a couple of days. It could also help to move your arm around after you get the shot, to diffuse the vaccine, and also to ask for the shot to be given in your non-dominant arm.