What is the Difference Between Heavy Chain and Light Chain?

The main difference between heavy chains and light chains in antibodies lies in their structure, composition, and function. Antibodies, also known as immunoglobulins, are multichain proteins consisting of two heavy chains and two light chains. Here are the key differences between heavy and light chains:

1. Structure: Heavy chains are composed of approximately 450 amino acids and have a molecular weight of about 50 kDa. Light chains, on the other hand, are smaller, with about 211-217 amino acids and a molecular weight of 25 kDa.
2. Constant Domains: Light chains contain only one constant domain (CL), whereas heavy chains contain either three or four constant domains (CH1, CH2, CH3, and sometimes CH4). The constant domains define the isotype of the antibody.
3. Variable Domains: Both heavy and light chains have variable domains (VH and VL, respectively), which are involved in antigen binding. The variable domains in both chains contain three complementarity-determining regions (CDRs).
4. Isotypes: There are five types of mammalian heavy chains (μ, δ, γ, ε, and α), which define the class of immunoglobulin: IgM, IgD, IgG, IgE, and IgA, respectively. In contrast, there are only two types of light chains: lambda (λ) and kappa (κ).
5. Antigen-binding Sites: An antibody molecule has two antigen-binding sites, each formed by the combination of a heavy chain variable domain and a light chain variable domain. This enables the antibody to cross-link antigens and to mediate various immune responses.

In summary, heavy chains and light chains are both essential components of antibodies, but they differ in structure, composition, and function. Heavy chains define the isotype of the antibody and contain more constant domains, while light chains are smaller and have only one constant domain. Both chains contribute to the antigen-binding sites in the antibody molecule.

Comparative Table: Heavy Chain vs Light Chain

The main differences between heavy and light chains in antibodies are their size, structure, and function. Here is a table summarizing the differences:

Heavy chains are larger and have a constant region composed of three tandem Ig domains (CH1, CH2, CH3) and a hinge region for added flexibility. They also have a variable region (VH) that differs depending on the B cell that produced it. There are five types of heavy chains: γ, α, δ, ε, and μ, which define the classes of immunoglobulins (IgG, IgD, IgA, IgM, and IgE).

Light chains, on the other hand, are smaller and have a constant region composed of a single Ig domain and a variable region (VL) with two successive domains (CL and VL). There are two types of light chains: λ and κ. The type of light chain does not determine the class of immunoglobulin.