An antibody molecule, also known as an immunoglobulin, is a Y-shaped protein composed of four polypeptide chains: two identical heavy chains and two identical light chains. Each chain contains constant regions and variable regions. The variable regions contain antigen-binding sites that recognize and bind to specific antigens.
The two heavy chains are linked by disulfide bonds and also by a disulfide bond to a light chain. The light chains are also linked by a disulfide bond. The region at the end of the variable domain of each chain is called the hypervariable region or the complementarity-determining region (CDR), which is responsible for the specificity of the antibody.
The stem of the Y-shaped molecule contains the constant regions of the heavy chains, which determine the class or isotype of the antibody (e.g., IgG, IgM, IgA, etc.). The antigen-binding sites are located at the tips of the Y-shaped molecule, where the variable regions of the heavy and light chains come together.
Overall, the structure of an antibody molecule allows it to specifically recognize and bind to antigens, leading to the activation of immune responses that can neutralize or eliminate pathogens.