fibrinoid

We’ve talked here before about the 6 types of necrosis. One type, fibrinoid necrosis, is sort of an outlier – it’s added at the end of the list of main types of necrosis as if it doesn’t quite belong in the list (Robbins calls it a “special” type of necrosis). The mechanism isn’t usually discussed, either – so the question here is probably one that’s on many students’ minds.

Q. I can see that in every type of necrosis there is some sort of cells dying but I can’t figure out why fibrinoid necrosis is included, since only Ag/Ab complexes are formed and attached to fibrin causing thickness…what the hell dies in this process?

A. I’ll just cut to the chase: in fibrinoid necrosis, it’s the vessel wall that is undergoing necrosis.

If you remember back to the beginning of pathology, when you discussed hypersensitivity reactions, you may vaguely recall talking about immune-complex-mediated hypersensitivity (it’s called a type III hypersensitivity reaction). Immune complexes can circulate and deposit themselves anywhere they want – either in tissues or in vessel walls.

That’s what’s going on here. For some reason, the body forms antigen-antibody complexes, and those complexes stick to vessel walls. They don’t just sit there by themselves, though – they attract inflammatory cells and activate complement. All of this inflammation does a lot of damage, and eventually, the vessel wall becomes necrotic. Check out the pink, smudgy, inflamed vessel wall in the image above.

The “fibrinoid” part of the name seems to imply that fibrin has a central role in the mechanism of this type of necrosis. Well, it doesn’t. Yes, there is some fibrin present, but the main thing is those immune complexes and all the inflammation they cause.

The name came about because of the bright pink smudgy appearance of the vessel wall (which is due to necrotic tissue plus immune complexes and all the other stuff that accumulates in this reaction). It looks like there is a lot of fibrin present (fibrin is bright pink when you see a bunch of it in a tissue) – so this type of necrosis was called “fibrinoid.” Not the best choice for a name, but there you go.