On the prior page, you were looking at a photo of cells in a mouse. 
You may have guessed that those large green cells were part of the immune system, as you watched it engulf the smaller red circle. In fact most organisms have some types of cells that provide a defense against invaders. A general term for these cells is phagocytes, which comes from the Greek root phagein, "to eat" or "devour", and "-cyte", the suffix in biology denoting "cell."
Phagocytes were discovered by Elie Metchnikoff in the 1880s. Watch the video below to see his experiments that led to discovering them.
sea star phagocytes original exp from Lauren Levites on Vimeo.
