Answer:
Constant region
Step-by-step explanation:
Most of the antibodies consist of four polypeptide chains: two heavy (H) chains and two light (L) chains. The tips of the heavy and light chain make the variable regions (V) which in turn serve as the antigen-binding site. The rest part of the heavy and light chain constitutes the constant region (C).
Constant region (C) of the heavy chains is the same in all the antibodies of the same isotype but differs between the different classes of antibodies. Each antibody class posses unique constant region segment in their heavy chains with a specific function. For example, the constant region segment of heavy chains of IgG monomers serves as a binding region for phagocytes while that of IgE antibodies bind to mast cells and basophils.