Kinetics of the Reaction between a Monoclonal IgM Anti-I and Rabbit Red Cells

Rabbit red cells were found to bind a maximum of 1.8 × 10⁵ IgM anti-I-molecules. A K-value of 5.0 ± 0.8 × 10⁸ 1/M was determined for the reaction between the cells and intact anti-I at 5°C; the K-value was little affected by changes in temperature. Hybrid 17S and 8S molecules composed of varying amounts of cold agglutinin and inactive IgM were prepared by reduction with 2-mercaptoethanol and reassociation. The results obtained with the reassociated products indicate random reassociation at the half subunit level. It is concluded that a 17S fraction with four or more active half subunits shows binding similar to intact IgM, while 8S fractions with two active half subunits show binding similar to the cysteine-reduced 8S IgM.