Demonstration of Antibodies to Muscle by Absorption of Rheumatoid Factor

Samples of skeletal muscle tissue were treated with normal human serum and with sera from 25 patients with myasthenia gravis. Rheumatoid factor‐containing serum was absorbed with the washed tissue sediments. Waaler's test was performed before and after the absorption. Tissue treated with normal serum did not reduce the titre of rheumatoid factor. Tissue treated with sera from 21 patients with myasthenia gravis, gave a significant titre fall. Four sera from myasthenia gravis patients contained rheumatoid factor. Tissue treated with these, sera did not bind isologous rheumatoid factor. When the sera were treated with mercaptoethanol before incubation with the tissue sediments, absorption both of isologous and of autologous rheumatoid factor was demonstrated. Thus, absorption of rheumatoid factor allows discrimination between γ‐globulins bound to muscle tissue by the Fab portion or the Fc portion.