It is the first immunoglobulin to be produced after the immune response takes place. IgM is the predominant isotype in the primary response. IgM is the second most abundant immunoglobulin in the serum, after IgG. It represents 10-12% of the total porcine immunoglobulin. Its serum concentration is between 1 and 5 mg/ml; in milk, 0.3 - 0.9 mg/ml and in colostrums 2.5 - 3.2 mg/ml.