What is the Difference Between Heavy Chain and Light Chain

The key difference between heavy chain and light chain is that heavy chain is the large polypeptide subunit of an antibody, while light chain is the small polypeptide subunit of an antibody.

An antibody is an immunoglobulin. It is a large Y-shaped protein used by the immune system to identify and neutralize foreign proteins such as antigens of pathogenic bacteria and viruses. There are five types of mammalian antibody isotypes: IgA, IgD, IgE, IgG, and IgM. A typical antibody is composed of two immunoglobulin heavy chains and two immunoglobulin light chains. Therefore, heavy chain and light chain are two subunits of an antibody.

CONTENTS

1. Overview and Key Difference
2. What is Heavy Chain  
3. What is Light Chain
4. Similarities – Heavy Chain and Light Chain
5. Heavy Chain vs Light Chain in Tabular Form
6. Summary – Heavy Chain vs Light Chain

What is Heavy Chain?

Heavy chain is the large polypeptide subunit of an antibody. It is denoted as IgH. A typical antibody is composed of two immunoglobulin-heavy chains. Heavy chains are encoded by the gene loci that are located on chromosome 14 in the human genome. There are several types of heavy chains which define the class or isotype of an antibody. Moreover, these heavy chain types vary between different animals. All heavy chains contain different immunoglobulin domains. A heavy chain is usually with one variable domain (V_H). The variable domain is very important in antigen binding. A heavy chain also has several constant domains such as C_H1, C_H2, etc.

In B cell maturation, the production of a viable heavy chain is a key step. If the heavy chain is able to bind to a surrogate light chain and move to the plasma membrane of the B cell, then the developing B cell can begin producing its light chain. Furthermore, in mammals, there are five types of heavy chains: γ, δ, α, μ and ε. These different heavy chains define the classes of immunoglobulins; IgG, IgD, IgA, IgM, and IgE, respectively. Heavy chains α and γ have approximately 450 amino acids. On the other hand, heavy chains μ and ε have approximately 550 amino acids.

What is Light Chain?

Light chain is the small polypeptide subunit of an antibody. A typical antibody is composed of two immunoglobulin light chains. In humans, there are two types of light chains: kappa (K) chain, which is encoded by immunoglobulin kappa locus (IgK) on chromosome 2, and lambda (λ) chain, which is encoded by the immunoglobulin lambda locus (IgL) on chromosome 22.

Antibodies are generally produced by B lymphocytes, where each expresses only one class of light chain. Once the light chain class is set, it remains fixed for the life of the B lymphocyte. In healthy individuals, the total kappa to lambda ratio is roughly 2:1 or 1:1.5 in serum. A highly divergent ratio is indicative of neoplasm. Furthermore, only one type of light chain is present in a typical antibody. Each light chain is composed of two tandem immunoglobulin domains: one constant (C_L) domain and one variable domain (V_L). The variable domain is important for antigen binding. The length of a light chain protein is approximately 211 to 217 amino acids.

What are the Similarities Between Heavy Chain and Light Chain?

• Heavy chain and light chain are two subunits of an antibody.
• Both chains are polypeptides made up of amino acids.
• They are present in humans as well as other animals.
• The variable domains of both chains bind with antigens.
• Both are highly important parts of the antibody function.

What is the Difference Between Heavy Chain and Light Chain?

Heavy chain is the large polypeptide subunit of an antibody, while light chain is the small polypeptide subunit of an antibody. Thus, this is the key difference between heavy chain and light chain. Furthermore, there are five different types of heavy chains such as γ, δ, α, μ, and ε while there are two different types of light chains such as kappa (K) and lambda (λ).

The below infographic presents the differences between heavy chain and light chain in tabular form for side by side comparison.

Summary – Heavy Chain vs Light Chain

Antibody or immunoglobulin is a large Y-shaped protein that is used by the immune system to identify and neutralize foreign proteins named antigens. Heavy chain and light chain are two subunits of an antibody. Heavy chain is the large polypeptide subunit of an antibody, while light chain is the small polypeptide subunit of an antibody. So, this summarizes the difference between heavy chain and light chain.

Reference:

1. “Heavy-Chain Antibody.” An Overview | ScienceDirect Topics.
2. Janeway, Charles A, and Jr. “The Structure of a Typical Antibody Molecule.” Immunobiology: The Immune System in Health and Disease. 5th Edition., U.S. National Library of Medicine.

Image Courtesy:

1. “AntibodyChains” By Original uploader was Yohan at fr.wikipedia, translated to English and converted to .svg by Sbmehta (CC BY-SA 3.0) via Commons Wikimedia
2. “Antibody” (CC BY-SA 2.5) via Commons Wikimedia