What is the difference between IgG and IgM antibodies?

IgM antibodies are the first antibodies produced when the body is fighting an infection. IgM antibodies work to eliminate pathogens in the early stages of B-cell mediated (humoral) immunity before there is sufficient IgG. IgG is the most abundant immunoglobulin in circulation and provides the majority of antibody-based immunity against invading pathogens.