What happens when a vaccine enters the booldstream?

When a vaccine enters the bloodstream, it introduces antigens (weakened or inactivated parts of a pathogen) that stimulate the immune system. This prompts the body to produce specific antibodies and activate immune cells to recognize and fight the pathogen if encountered in the future. The immune system essentially "learns" how to respond, creating a memory that helps provide protection against future infections. Over time, the immune response wanes, but memory cells remain, allowing for a quicker response during actual exposure to the pathogen.