Antibodies are glycoproteins belonging to the immunoglobulin superfamily; the terms "antibody" and "immunoglobulin" are often used interchangeably. Antibodies are typically made of basic structural units—each with two large "heavy chains" and two small "light chains". There are several different types of antibody heavy chains, and several different kinds of antibodies, which are grouped into different "isotypes" based on which heavy chain they possess. Five different antibody isotypes are known in mammals. They help direct the appropriate immune response for each different type of foreign object they encounter.