Immunity and inflammation are almost as old as life itself.
Even amoebae, the single-celled organisms thought to be one of the first forms of life on Earth, are capable of distinguishing between members of their own species and other species they can eat. This capacity to distinguish "self" from "non-self" is what normally prevents our more complicated immune system from attacking our own tissues.
The cellular foundations of our immune system were laid at the time the first multi-cellular life forms, primordial worm-like creatures, wriggled their way along the ocean floor. These organisms developed primitive immune cells that held a constant vigil against invading germs. Prototypes of the more sophisticated germ-eating macrophages and dendritic cells of our advanced immune system, these cells provided a form of protection we now call "innate" immunity.
A major evolutionary milestone occurred in jawed fishes such as sharks and rays, which developed “adaptive” immunity, the ability of immune cells to generate diverse responses against pathogens. These responses include production of antibodies and stimulation of subpopulations of “T” lymphocytes that can seek out and destroy germ-harboring cells.
Adaptive immunity provides a survival advantage—the ability to "remember" a germ to which the human or animal host had previously been exposed, and to mount a bigger and stronger response the next time the pathogen invaded. But the cells that fight off attacking microbes also can damage normal tissue under circumstances that often involve chronic inflammation.
It seems evolution has handed us a loaded gun in our immune system and the inflammatory response. Without it, we would be defenseless. With it, we may just be shooting ourselves in the foot.