An Antibody is defined as “an immunoglobulin capable of specific combination with the antigen that caused its production in a susceptible animal.” Antibodies are produced in response to the invasion of foreign molecules in the body. Antibodies exist as one or more copies of a Y-shaped unit, composed of four polypeptide chains. Each Y contains two identical copies of a “heavy” chain, and two identical copies of a “light chain”, named as such by their relative molecular weights.
All antibodies are Immunoglobulins although not all Igs can function as antibodies. The function of an antibody is to bind its antigen as tightly as possible and then direct it towards other components of the Immune System so that it can be destroyed.
Antibodies can be made against proteins, carbohydrates, lipids and nucleic acids. Antibodies to nucleic acids and lipids can be found in autoimmune diseases. Antibodies to small organic molecules can cause allergies to drugs
Serological studies have led to the description of three important determinants on the antibody molecule. First, idiotopes reflect the uniqueness of structural features of the variable region determinants specific for an individual antibody, or idiotype. Second, allotopes are antibody characteristics reflecting allelic polymorphisms that differ between individuals of the same species. Third, isotopes designate antigenic determinants present on antibodies shared by all individuals of a given species.
In humans, five classes of immunoglobulins, also called isotypes (IgG, IgM, IgA, IgD and IgE), differ in their physicochemical and serological properties, and in the amino acid sequence of their constant regions. In their monomeric form, all classes exhibit the same basic structure: two heavy (H) and two light (L) chains. While IgG, IgD and IgE consist of a single monomeric unit, the IgM and IgA classes are composed of monomers that associate to form pentamers and dimers, respectively.
Formation of these polymers requires an additional polypeptide chain termed the J chain. Serological analysis of immunoglobulins of the same class revealed the existence of additional heterogeneity and resulted in the subdivision of classes into subclasses. In addition to the five major classes, there are four subclasses of IgG in humans (IgG1 to IgG4) and in mice (IgG1, IgG2a, IgG2b and IgG3), and two subclasses of IgA in humans (IgA1 and IgA2).
The bonding between antigens and antibodies is dependent on hydrogen bonds, hydrophobic bonds, electrostatic forces, and van der Waals forces. These are all bonds of a weak, non-covalent nature, yet some of the associations between antigen and antibody can be quite strong.
Like antibodies, antigens can be multivalent, either through multiple copies of the same epitope, or through the presence of multiple epitopes which are recognized by multiple antibodies. Interactions involving multivalency can produce more stabilized complexes, however multivalency can also result in steric difficulties, thus reducing the possibility for binding.
All antigen-antibody binding is reversible, however, and follows the basic thermodynamic principles of any reversible bimolecular interaction: KA = [Ab-Ag]/[Ab]*[Ag]
Where KA is the affinity constant, Ab and Ag are the molar concentrations of unoccupied binding sites on the antibody or antigen respectively, and Ab-Ag is the molar concentration of the antibody-antigen complex.
Avidity is a measure of the overall stability of the antibody-antigen complex. It is controlled by three major factors – the affinity of the antibody for the epitope, the valency of the antigen and antibody, and the structural arrangement of the interacting parts.
Increases in: Chronic granulomatous infections| Infections of all types | Hyperimmunization | Liver disease | Malnutrition (severe) | Dysproteinemia | Rheumatoid arthritis | Disease associated with hypersensitivity granulomas, dermatologic disorders, and IgG myeloma
Decreases in: Agammaglobulinemia | Lymphoid aplasia | Selective IgG, IgA deficiency | IgA myeloma | Bence Jones proteinemia | Chronic lymphoblastic leukemia
Increases (in adults) in: Waldenström’s macroglobulinemia | Trypanosomiasis | Actinomycosis | Carrión’s disease (bartonellosis) | Malaria | Infectious mononucleosis | Lupus erythematosus | Rheumatoid arthritis | Dysgammaglobulinemia (certain cases)
Note: In the newborn, a level of IgM above 20 ng./dl is an indication of in utero stimulation of the immune system and stimulation by the rubella virus, the cytomegalovirus, syphilis, or toxoplasmosis.
Decreases in: Agammaglobulinemia | Lymphoproliferative disorders (certain cases) | Lymphoid aplasia | IgG and IgA myeloma | Dysgammaglobulinemia | Chronic lymphoblastic leukemia
Increases in: Chronic infections | IgD myelomas
Increases in: Atopic skin diseases such as eczema | Hay fever | Asthma | Anaphylactic shock | IgE-myeloma
Decreases in: Congenital agammaglobulinemia | Hypogammaglobulinemia due to faulty metabolism or synthesis of immunoglobulins.