IgM antibodies 。Pentamer 。5-10%of serum antibodies Disulfide bond 。Fix complement 。In blood,ymph,onB cells J chain Agglutinates microbes; first Ab produced in response to infection 。Half-life=5days• Pentamer • 5-10% of serum antibodies • Fix complement • In blood, lymph, on B cells • Agglutinates microbes; first Ab produced in response to infection • Half-life = 5 days IgM antibodies