Immunoglobulin M (IgM) constitutes the natural haemagglutinins (anti-A and anti-B > BG-AB)

IgA is the predominant immunoglobulin in normal bronchial secretions (u.a. externen Körperflüssigkeiten)

Only IgG freely crosses the placenta (but requires a transport mechanism)

IgG has a molecular weight of 150,000 daltons and bears only two antigen-binding sites.