Some studies suggest that getting a booster in the same arm as the initial shot can boost the immune system’s response. Now, we’re getting clues as to why using the same arm for initial and booster doses might be better for activating the immune system’s response to a vaccine. The macrophages then direct where memory B cells go to more effectively respond to the booster when it is given in the same arm. Previously, the team from the study looked at memory B cells. They found that memory B cells tend to stay on the lymph node closest to the injection site.