Antibodies are Y shaped. The two heavy chains in a given antibody are identical and form the inside of the Y. Each heavy chain is about 450 amino acids long. The two heavy chains are held together by disulfide bonds and each heavy chain is connected to a light chain by another disulfide bond.
The two light chains in a given antibody molecule are identical to one another. They are smaller than heavy chains and consist of only 250 amino acids. The light chains are found on the outside of the heavy chains, extending only through the arms of the Y shape, not it's base.
The constant region of an antibody is so names because it does not change between the antibodies of a given type. There are five different constant regions and they divide antibodies into five major classes: IgM, IgG, Iga, IgD and IgE. The constant region is made of both heavy and light chain components. It extends from the base of the Y shape up to the bottom half of the arms.
The variable region of an antibody is what binds to various invading microbes. It is composed of both heavy and light chain parts. It forms the top half of the arms of the Y-shaped antibody. This region varies from antibody to antibody and this diversity allows the immune system to recognize millions of different structures.