Difference Between Immunoglobulin and Antibody

Main Difference – Immunoglobulin vs Antibody

Immunoglobulin and antibody are disease-fighting proteins developed by most vertebrates in response to a particular antigen. Both immunoglobulin and antibody are glycoproteins. Both of them contain similar regions in their molecules. Immunoglobulins are attached to the B cell membrane while antibodies float in the circulation. The main difference between immunoglobulin and antibody is that immunoglobulin has a transmembrane domain in order to be attached to the plasma membrane whereas antibody does not have a transmembrane domain. The five immunoglobulin classes are IgG, IgM, IgA, IgD, and IgE. An antibody is a Y-shaped glycoprotein. Immunoglobulins are also called surface immunoglobulins. Both immunoglobulin and antibody are components of the immune system.

Key Areas Covered

1. What is an Immunoglobulin
      – Definition, Structure, Function
2. What is an Antibody
      – Definition, Structure, Function
3. What are the Similarities Between Immunoglobulin and Antibody
      – Outline of Common Features
4. What is the Difference Between Immunoglobulin and Antibody
      – Comparison of Key Differences

Key terms: Antibody, Antigen, B Cells, Constant Domain, Immune System, Immunoglobulin, Pathogens, Transmembrane Domain, variable Domain

What is an Immunoglobulin

Immunoglobulin refers to any class of structurally related proteins in the serum and the cells of the immune system which functions as antibodies. It is produced in response to an antigen. The name immunoglobulin is derived from the fact that they migrate with globular proteins when antibody-containing serum is placed in an electrical field. The five classes of immunoglobulins are IgG, IgM, IgA, IgD, and IgE. The structure and the function of five immunoglobulin classes are shown in figure 1.

Main Difference - Immunoglobulin vs Antibody

Figure 1: Classes of Immunoglobulins

Immunoglobulins are structurally similar to antibodies. That means immunoglobulins contain a Y-shape with two heavy and light chains. Immunoglobulins are found attached to the plasma membrane of the B cells, which produce immunoglobulins. In order to be attached to the plasma membrane, immunoglobulins should possess a transmembrane domain as well.

What is an Antibody

An antibody refers to a globin protein, which is produced by B cells in response to a particular antigen. The most characteristic feature of antibody is its specificity to a particular antigen. Typically, antigens are Y-shaped molecules. They are made up of two identical heavy chains and two identical light chains. The four-chain structure is held together by disulfide bonds between the chains. Both heavy and light chains contain variable and constant regions. The amino acid sequence of the constant region is conserved among the antibodies, but the amino acid sequence of the variable region can be different to each other. The arms of the antibody molecule are formed at the hinge region, giving a Y shape to the molecule. The variable region gives specificity to antibodies. The structure of a typical antibody is shown in figure 2.

Difference Between Immunoglobulin and Antibody

Figure 2: Antibody

Antigen binding and effector function are the two functions of immunoglobulins. Immunoglobulins can bind to a specific antigen determinant on the surface of pathogens such as virus , bacteri a, fungi, and parasites or pathogen-infected cells in the body. The binding of antibodies to a pathogen may either neutralized the pathogen. Antibodies exhibit two effector functions: fixation of the complement and binding to various cell types. The binding of antibodies to a particular pathogen may induce the complement system to destroy the pathogen. The antibody-bound pathogens may induce the immune system cells such as macrophages, mast cells, and lymphocytes to trigger an immune response against them.

Similarities Between Immunoglobulin and Antibody

Both immunoglobulin and antibody are disease-fighting molecules produced by the immune system of most vertebrates.
Both immunoglobulin and antibody comprise heavy and light chains.
Both immunoglobulin and antibody comprise variable and constant regions.
Both immunoglobulin and antibody are produced in response to the presence of an antigen.
Both immunoglobulin and antibody can be found in secretions as well as in the circulation.
Both immunoglobulin and antibody are involved in protecting the body from pathogens.

Definition

Immunoglobulin: Immunoglobulin refers to any class of structurally related proteins in the serum and the cells of the immune system which functions as antibodies.

Antibody: Antibody refers to a globin protein which is produced by B cells in response to a particular antigen.

Location

Immunoglobulin: Immunoglobulin occurs on the surface of B cells.

Antibody: Antibody freely occurs in the circulation.

Transmembrane Domain

Immunoglobulin: Immunoglobulin comprises a transmembrane domain in order to be attached to the plasma membrane of B cells.

Antibody: Antibody does not have transmembrane domains.

Classes

Immunoglobulin: The five immunoglobulin classes are IgG, IgM, IgA, IgD, and IgE.

Antibody: A particular antibody type is specific to a particular pathogen.

Function

Immunoglobulin: The function of immunoglobulin depends on the type of heavy chain.

Antibody: The non-self antigens are recognized by specific antigens and are neutralized by antibodies.

Conclusion

Immunoglobulin and antibody are two types of glycoprotein molecules which are produced in response to a specific antigen. Immunoglobulins are always attached to the plasma membrane of the B cells. But antibodies can be found freely in the circulation. The main difference between immunoglobulin and antibody is the occurrence of each type of molecule in the body.

Reference:

1. “Immunoglobulins: structure and functions.” The Biochemistry Questions Site, 26 May 2009, Available here.
2. Mandal, Ananya. “What is an Antibody?” News-Medical.net, 3 Aug. 2017, Available here.

Image Courtesy:

1. “Antibody” (CC BY-SA 2.5) via Commons Wikimedia
2. “2221 Five Classes of Antibodies new” By OpenStax College – Anatomy & Physiology, Connexions Web site. Availabe here, Jun 19, 2013. (CC BY 3.0) via Commons Wikimedia