Final answer:
The Fc portion of an antibody is formed by the lower portion of the heavy chains, enabling immune system interactions through Fc receptors.
Step-by-step explanation:
The Fc portion of an antibody is formed by (C) the lower portion of the heavy chains. Antibodies, or immunoglobulins, are glycoproteins made up of two types of polypeptide chains, referred to as heavy and light chains. Each antibody molecule is composed of two heavy chains and two light chains that are held together by disulfide bonds to form a four-chain structure. The Fc region is essential for the antibody's interaction with immune cells, as it can bind to Fc receptors present on the surface of these cells, allowing them to recognize and eliminate antibody-coated pathogens. This region is found in the constant portion of the antibody molecule, constituting the trunk of the 'Y' shaped structure, and is a critical element in immune response mechanisms such as opsonization and activation of complement factors.