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Also known as specific immunity, active immunity is carried out by T & B lymphocytes and reacts to specific antigens of cells; active immunity also produces memory cells to target the same pathogen if we are ever exposed to it again.
A unique marker that has the potential to stimulate an immune response; antigens are basically the identity marker of cells for the immune system to see.
Not every vaccine has lifelong effects, so when antibody levels decline after a certain period of time a booster shot is given to bring the back up again.
A therapy that involves injecting an augmented/altered pathogen into a person to induce active immunity; vaccines prepare a person for when they are naturally exposed to specific pathogens by stimulating them to already have the proper antibodies and memory cells in their systems.