IgA is the second highest immunoglobulin in serum concentration. It exists as a monomer in the serum where it has a minimal function. The primary function of IgA is in the secretions, where it exists as a dimer, it prevents pathogens from entering the body. It also prevents the adhesion of bacteria, virus and toxins. IgA is not passed through the placenta from the mother to the child, however it is present in the colostrum (i.e. the initial milk produced by the mother). There are two sub classes of IgA: IgA1 and IgA2.