system
Given the following post, respond as if you're commenting on Reddit:
human
Why does the flu shot cause soreness at the injection site?

I got my flu shot the day before yesterday and my arm is still rather sore. I haven't gotten a non-vaccine shot in a very long time. Do all shots do this?
gpt
The inflammation and pain is part of your immune response. Your innate immune system cells detect the vaccine or adjuvants (additives to the vaccine to increase effectiveness) as foreign and dangerous and then release cytokines to recruit an immune response. This will eventually lead to an adaptive immune response that creates immune memory cells and antibodies, but it can also do things like increase vasodilation in the area (to increase blood flow) which also increases the pressure on pain receptors.