Direct visualization of IgM antibodies bound to tissue antigens using a monoclonal anti-type III collagen IgM as a model system. 
A mouse monoclonal IgM antibody directed against human Type III collagen was utilized to immunolocalize Type III collagen by transmission and scanning electron microscopy without the use of an electron-dense conjugate.  Because bound IgM can be directly visualized, primary or secondary antibody conjugates, such as ferritin, HRP, colloidal gold, etc.,  are unnecessary in this method.  Immunolocalization to Type III collagen in the matrix of human skin and to fibrils formed in vitro using only IgM antibody reveals uninterrupted IgM binding which exactly matches the banding period of the collagen fibrils.  In contrast, colloidal gold-conjugated secondary antibody complexes directed against primary IgM binding sites reveal less precise labeling.  The data suggest that direct visualization of primary monoclonal IgM antibodies may be useful in a wide variety of highly specific ultrastructural immunolocalization studies without requiring the use of electron-dense conjugates.