And so that calls upon macrophages, natural killer cells, cytotoxic, T cells and many other different types of white blood cells to that area and that destroys that pathogen along with its antigen. Now, this is basically how our immune system defends and protects our body from these invading pathogens. Now, following our infection, our body develops something called active immunity. And what that means is our body develops these memory cells. And these memory cells contain a copy of the antibody that is specific to that antigen that infected our body in the first place. And what that is useful for.