The immunoglobulins, or antibodies, found in colostrum or milk are the same as those found in the blood or mucosal secretions. They are a family of proteins with a range of protective bioactivities. Immunoglobulins are divided into several classes including IgM, IgA, IgG, IgE, and IgD [114] , and IgG, IgA and IgM are the major immunoglobulin classes in mammary secretions. IgM is the class that appears initially when an organism is exposed to an antigen for the first time (primary infection). IgM has a low specificity and hence a lower potency in defeating the infection. IgA is the major immunoglobulin class found in mucosal secretions and prevents mucosal infections by agglutinating microbes, whereas IgG is the primary immunoglobulin class found in bovine colostrum and milk. Several subclasses of IgG exist, with IgG1 and IgG2 being the major immunoglobulins in serum. All monomeric immunoglobulins have the same basic molecular structure, being composed of two identical heavy chains and two identical light chains, with a total molecular mass of approximately 160 kilodaltons (for details on immunoglobulin structure see [5, 14, 16] ). Both the heavy and light chains have constant regions and variable regions. Heavy and light chains are linked together by disulfide bonds, resulting in the classic Y-shape of the immunoglobulin molecule [114] . The number and location of the disulfide bonds is dependent on the class of immunoglobulin. Each immunoglobulin molecule has two antigen binding sites which comprise the antigen-binding fragment (Fab). The Fab includes the variable amino acid domain. At the other end of the molecule is the constant fragment (Fc) which has a constant amino acid sequence among molecules of the same subclass and which confers the identity of an immunoglobulin as a particular subclass. The Fc region of the molecule is the region that binds to Fc receptors on various cell types.