IgG is the main isotype in the swine species. It represents between 80 to 85% of total immunoglobulins in serum and colostrum. It is the most important antibody in the secondary response. Its serum concentration is from 17-29 mg/ml, in milk, 1-3 mg/ml and from 30-70 mg/ml in colostrums. At least five subclasses have been described: IgG1, IgG2a, IgG2b, IgG3 and IgG4. DNA studies have shown however, that there are 8 genes encoding the constant region Cg.