Abstract
Free IgM immunoglobulins were examined in the electron microscope using the negative contrast technique. Normal human and rabbit IgM and Waldenström macroglobulins were indistinguishable from one another and revealed flexible spider-like particles with five appendages joining a central ring. The average total span of the molecules was 300 A. The appendages were about 125 x 30 A; the central ring had an outer diameter of approximately 100 A and an inner diameter of 40 A. Some purified 19S IgM preparations tended to form massive aggregates (>/=50S) which, when examined in the electron microscope, revealed enormous clumps of IgM molecules whose appendages were entangled with one another. Electron microscopy of reduced-alkylated IgM revealed total absence of intact spider-like molecules. The predominating structure observed was a round electron-dense knob about 50 A in diameter which in some cases had a fine fiber-like extension with approximate dimensions 100 x 15 A. Rabbit and human IgM molecules with antibody activity to poliovirus dried in sodium tungstosilicate on a carbon film as in preparation for electron microscopy were shown to retain nearly 100% of their poliovirus neutralizing activity after redissolving in a physiological buffer.