In the case of polymeric immunoglobulins, including the polymeric forms of IgA and IgM that are found in milk, the monomeric forms of the immunoglobulins are linked together through the covalent interaction with a joining (J) chain [114, 115] . The result is a dimeric form of IgA and a pentameric form of IgM. Binding of these immunoglobulins to the J chain also results in them having several special features, including: a high valency of antigen-binding sites, allowing them to agglutinate bacteria; limited complement-activating activity, which allows them to act in a noninflammatory manner; and a high affinity for the polymeric immunoglobulin receptor (pIgR) that is responsible for transepithelial transport of IgA and IgM into mucosal secretions such as milk [116] . The pIgR and its relationship to the secretory component (SC) associated with secretory IgA and secretory IgM is discussed further below (section 5.2).