MOPC-21: MOPC-21 immunoglobulin has unknown specificity. The isotype of this antibody is IgG1,κ. MOPC-173: MOPC-173 immunoglobulin has unknown specificity. The isotype of this antibody is IgG2a,κ. Both MOPC-21 and MOPC-173 immunoglobulins were chosen as isotype controls after screening on a variety of resting, activated, live, and fixed rat and human tissues.