Immune cells like macrophages and dendritic cells patrol the body to identify foreign invaders such as viruses, bacteria, and fungi, and present them to immune cells in lymph nodes.

“In addition to that, you have cells that go out into your body, things like macrophages, things like dendritic cells, okay, the names don't matter so much, that are looking for foreign invaders. Again, it could be viruses, bacteria, funguses. It could be even just physical bodies that somehow made it into your system one way or the other. And they will literally present things to the T cells and B cells, and other immune cells within the lymph nodes, so that the T cells and B cells can generate, through a process, this takes some time, to generate antibodies to fight those things off.”