Neutrophil vs. monocyte

Here’s an example of a common question students have in the beginning of a medical school or dental school pathology course.

Unfortunately, students often feel like they “should” know the answers to certain questions – so they don’t ask. Don’t fall into this trap! You never need to feel embarrassed about asking a question; everyone has things they don’t know – even professors. That’s why you’re taking the class – to learn!

On to the question.

Q. What is the main difference between a neutrophil and a monocyte? This is what I understand:

PMNs:
fight bacteria and fungi (but they are different than NK cells–right?)
act as antigen presenting cells
phagocytic
are generally the first to arrive; part of the acute inflammatory response

Monocytes:
act as antigen presenting cells
can secrete cytokines and attract inflammatory cells like fibroblasts, etc.
phagocytic
bigger role in chronic inflammation

A. Broadly, the similarities are: neutrophils and monocytes are both phagocytes, and they both work to fight infections. But moncytes can turn into macrophages (when they get into tissues), which are very good at eating things, as well as presenting antigens. Neutrophils eat, but don’t present, antigens. One of the big differences, too, you already mentioned: neutrophils are the first to come in during an inflammatory process. Lymphocytes come next, then monocytes/macrophages come in to mop up the mess.

One note: neutrophils are phagocytes, but not antigen presenting cells. Another note: You are right, neutrophils are different than NK cells. NK (natural killer) cells are specialized lymphocytes which have functions different than those of neutrophils and monocytes.

Also: neutrophils look different than monocytes/macrophages. Neutrophils have a “busy” nucleus (that’s why they are called “polymorphonuclear” leukocytes), with several lobes. You can see one at 2 o’clock in the above photo. They also have granules, both primary (azurophilic) and secondary (fawn-colored). Monocytes have a horseshoe-shaped nucleus, with dishwater-gray cytoplasm and a few tiny granules. See the lower left corner in the above photo.