An antibody molecule is basically shaped like a Y. The molecule has two parts: Variable part: This part varies from antibody to antibody, depending on which antigen the antibody targets. The antigen attaches to the variable part. Constant part: This part can be one of five structures, which determines the antibody’s class— IgM, IgG, IgA, IgE, or IgD. This part is the same within each class.