The inset to the upper left is what coronavirus actually look like when seen through a  Transmission Electron Microscope,  (magnified about 200,000 times). The virus' jeweled crowns (coronas) give them their name. The spiked blue oval to the lower right of your screen, with a corkscrew strand of RNA within it, is my fanciful conception of what one of those coronavirus might look like. The spikes are the receptors that these virus uses to attach to and enter, their targeted cells. It appears that the proteins that form these spikes must mutate in order for feline corona virus to cause FIP. (ref)

I drew three pink macrophages similar to the way they would look when viewed through a Scanning Electron Microscope (7,000 times larger than they actually are). Macrophages play a key role in the severe inflammation that occurs in FIP. In the vast majority of cats with coronavirus, the virus is content to invade the cells that line the pet's lower intestines causing only mild, temporary symptoms or no symptoms at all.

But when feline coronavirus makes its ominous transformation to the "wet" or "dry" form of FIP, the virus is no longer content in the cat's intestine. It begins to invade the macrophages that protect your cat's other organs. Sometimes that occurs primarily in the pet's chest (lungs); in others it is the surface of its abdominal organs that become inflamed , in some cats, it is both and in some cats a slower inflammation of body organs occurs (the "dry" form).

Monocytes and macrophages are the foot-soldiers you cat uses to protect itself from foreign invaders. They are covered with ever-changing strandlike tentacles (pseudopodia) that engulf and cleanse the body of invading germs, cellular debris and substances the body's immune system deems as unwanted or dangerous. Ironically, in FIP, these hunter cells have become the prey.