One thing that’s hard to get the hang of in pathology is the difference in appearance between chronic and acute inflammation in tissue sections. It’s pretty easy to tell a neutrophil from a lymphocyte in blood smears – but it’s a different story in tissue sections. Someone will show a low-power image in lecture and say it’s obviously chronic inflammation – but how are you supposed to be able to recognize that?

The most helpful idea to keep in mind, I think, is how “busy” the collection of cells looks. If you think about the morphologic differences between neutrophils and lymphocytes, the most obvious one is the nucleus. The nucleus of a neutrophil has several lobes (which is why it’s sometimes called a polymorphonuclear cell), but the nucleus of a lymphocyte is a single, large, dark circle.

When you look at a bunch of neutrophils in section, you’ll see a lot of cells with multi-part nuclei that look like little Mickey Mouse ears. Check out the upper photo. Acute inflammation looks messy, or busy, like a bunch of dots of all different sizes.

When you look at a bunch of lymphocytes in section, though, you’ll see a bunch of similarly-sized, uniform, dark blue dots. Check out the lower photo. Chronic inflammation looks more uniform, like somebody had a rubber stamp of a little purple dot, and they just stamped it over and over on the slide.

I kinda want to clean up the top photo.