The document discusses the structure of immunoglobulins (antibodies). It describes how immunoglobulins are Y-shaped molecules composed of two heavy chains and two light chains that form antigen-binding and crystallizable fragments. The variable regions of the heavy and light chains form antigen-binding sites and contain hypervariable regions that provide antibody diversity. The constant regions define the antibody class and confer effector functions. The hinge region links the Fab and Fc fragments. The document also lists the different antibody classes determined by the heavy chain constant region.