The difference between an antigen and an antibody is that an antigen is a collective term for all substances that induce an immune response in oneself, while an antibody is a collective term for a class of immunoglobulins produced by oneself that bind specifically to an antigen. 1. Antigens (Ag) can be recognized by B cells and activate T cells, thus triggering a continuous immune response. Antigen has the basic properties of specificity (an antigen can only bind specifically to the corresponding effector T cell or antibody), macromolecule (relative molecular mass is more than 10,000, and antigenicity increases with molecular weight), and xenobiotic (can not be accepted by its own tissue cells), etc. Antibodies are immunoglobulins with protective effects. 2. Antibodies are proteins that have a protective effect and are produced mainly for the stimulation of antigens. Antibodies are secreted by plasma cells (effector B cells), which can specifically recognize antigens, and are mainly used to identify and neutralize antigens in the immune system, with five main types: IgG, IgM, IgA, IgD, and IgE. The main functions of antigens are to prevent the invasion of pathogens and neutralize toxins; to activate complement and induce cytolytic destruction; to regulate phagocytosis and cytotoxicity, and to mediate type I hypersensitivity reactions.