In the article "Rheumatoid: more than just a single disease", the author commented that inflammatory blood markers (ESR and CRP) are important because they show that RA is more than a joint disease. These markers show that inflammation in present in blood stream (serum) which demonstrates that it is a systemic disease.
I liked the way this was worded because I've often come across people who, I feel, diminish our suffering by not recognising the serious nature of RA. This is not, of course, to belittle the struggle of those who do have purely muscular skeletal conditions. I thought the explanation was nice and simple and might be helpful to show others, if you feel they are not fully understanding your struggles.