What is the difference between inflammation and irritation?

Question

I wonder what the difference is between inflammation and irritation, especially in the context of tendinopathies.

My understanding based on MedlinePlus and Wikipedia is that inflammation is one kind of inflammation.

I sometime see the two terms employed synonymously, e.g. in this Healthline article:

Tendons are thick cords that join your muscles to your bones. When these tendons become irritated or inflamed, it is called tendinitis. This condition causes acute pain and tenderness, making it difficult to move the affected joint.

or this Johns Hopkins University article:

Allergic conjunctivitis is inflammation or irritation of the clear mucous membrane lining the inner eyelids and sclera. This lining is called the conjunctiva.

In the context of tendinopathies, I would tend to believe they are synonymous as I cannot think of any other type of irritation other than inflammation, but I am not sure.

Answer 1

The following is derived from my education (MD).

In the medical field, inflammation has a specific meaning even though its use is broad. Whenever you see a word ending with -itis, the suffix represents inflammation. Inflammation is a response from your body to some kind of trigger (such as infection, trauma, against yourself (autoimmune), et cetera). Inflammation is defined usually to have the "4 -or's" which are the following:

1. Rubor - redness
2. Dolor - pain
3. Tumor - increase in size (swelling)
4. Calor - heat

These are caused by the inflammatory response which heavily includes immune cells.

Irritation is not a medically well defined word. We use it to mean something is just responding to a trigger. We use it often to describe inflammation in layman's terms. It is not something you can look too much into. Any dictionary definition will do. The connotation is usually negative (so it is a bad thing in general).