rheumatoid factor
noun
: an autoantibody of high molecular weight that reacts against IgG immunoglobulins and is often present in rheumatoid arthritis
Example Sentences
Recent Examples on the Web Nearly everyone who is evaluated for psoriatic arthritis will have their blood tested for rheumatoid factor (RF), a protein produced by the immune system that attacks healthy joints. 
Sara Gaynes Lev, SELF, 30 July 2021 The doctor on call had the presence of mind to do a rheumatoid factor test, which tests for a type of protein that your immune system produces that can attack healthy tissue. Natasha Lavender, SELF, 15 Oct. 2020
Sara Gaynes Lev, SELF, 30 July 2021 The doctor on call had the presence of mind to do a rheumatoid factor test, which tests for a type of protein that your immune system produces that can attack healthy tissue. Natasha Lavender, SELF, 15 Oct. 2020 Word History
First Known Use
1949, in the meaning defined above
Medical Definition
rheumatoid factor
noun
: an autoantibody of high molecular weight that reacts against immunoglobulins of the class IgG and is often present in rheumatoid arthritis