The IgM molecule is the oldest class of immunoglobulins, and it is a large molecule consisting of five basic units held together by a J chain. The major role IgM plays is the intravascular neutralization of organisms, especially viruses. The reason for this important physiological role is that it contains five complement-binding sites, resulting in excellent complement activation. This activations permits the segment removal of antigen-antibody complement complexes via complement receptors on phagocytic cells or complement-mediated lysis of the organism. However, in contrast to the IgG molecule, it has relatively low affinity binding to the antigen in question. Second because of its size, it does not usually penetrate into tissues.