This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Microbiology
Which of the following is TRUE about rheumatoid factor? 
Select and explain. 
A. An albumin, B. 17S globulin, C. 7S globulin, D. 19S globulin
D: 19S globulin
Answer: D: 19S globulin 
Explanation: Rheumatoid factors (RFs) are autoantibodies, predominantly IgM isotype (19s IgM), that are directed against multispecies antigenic determinants on the heavy chain of IgG. RFs can arise as an epiphenomenon during acute illness or chronic antigenic stimulation of almost any cause. They are present, in significant titer, in bacterial endocarditis, granulomatous diseases, and most rheumatic diseases. Ref: Textbook of Microbiology By Ananthanarayan And Paniker, 2005, Page 174.