: IgM is the largest of the immunoglobulins and the first to reach the site of infection being present in the circulating fluid. When a B- lymphocyte is first stimulated by antigen IgM is the first antibody type to be produced during an immune response. IgM fixes complement and does not cross placenta. Later on IgG replaces IgM . IgG is the main antibody in the secondary response. It crosses the placenta. IgA is the predominant antibody class in seromucous secretions such as saliva, tears, colostrums and respiratory, gastrointestinal and urinogenital secretions. IgA provides the first line of defence against invading pathogens, inhibiting their adherence to the mucosal cells and to preventing their invasions of the body.