Final answer:
The J-Chain is the molecule that connects multiple IgM and IgA antibodies to form multimers, enhancing their ability to bind to antigens through increased avidity.
Step-by-step explanation:
The molecule that holds multiple molecules of IgM and IgA together is known as the J-Chain. This is a 15 kD polypeptide that is disulfide bonded to the tailpieces of these antibodies, thereby stabilizing their multimeric structures. Specifically, IgM can form pentamers, with five antibody molecules being connected, and IgA is typically found as dimers in secretions. The structure of IgA and IgM involves multivalent binding sites, which enhance their avidity, meaning that even though they may have lower affinity than monomeric antibodies, they can bind to many antigens simultaneously to effectively combat pathogens.