Immunoglobulin | What is an antibody?

Immunoglobulins (antibodies) are glycoproteins naturally produced in response to invading foreign particles (antigens) such as micro-organisms and viruses. They play a critical role in the immune system’s defense against infection and disease.

Immunoglobulin - what is an antibody?

Antigens recognized and bound by immunoglobulins could be proteins and/or sugars on bacterial and viral cell surfaces, but they may also be distinctive molecules found expressed on cancerous cells. The region of an antigen that interacts with an antibody is termed the epitope. 

Typically, an animal’s immune system will generate a large group of immunoglobulins that recognize several epitopes of a particular antigen. Each immunoglobulin is secreted by a different antibody-producing plasma cell. As the antibodies found in serum are collectively produced by many plasma cells (clones), they are described as polyclonal. While this is an advantage for fighting infections in nature, the heterogeneity of polyclonal antibodies limits their use as research tools.

A major breakthrough in the development of immunoglobulins for research applications was the production of monoclonal immunoglobulins in 1975 by Köhler and Milstein (1). These are produced by removing B cells (plasma cell precursors) from the spleen of an animal that has been challenged with an antigen. The B cells are subsequently fused with an immortal myeloma tumor cell line. This results in a single-cell hybrid known as a hybridoma. The B cells confer antibody production capability, while the myeloma cells enable hybridomas to divide indefinitely and grow well in culture. Since the hybridomas secrete only one antibody type, they are monoclonal, effectively ensuring an infinite supply of this immunoglobulin selective for a single epitope.

Table 1: Features of Monoclonal and Polyclonal Immunoglobulins

Monoclonal Antibody

Polyclonal Antibody

Consists of one immunoglobulin class/subclass which is selective for a single epitope on the antigen

Contains a mixture of antibodies (mainly IgGs), often recognizing multiple epitopes on the antigen

Usually generated in mice, other rodents, or isolated from a phage display library

Generated in a variety of species including rabbit, goat, sheep, and donkey

Always identical, as they are produced from the same hybridoma (or transfected cell line, or bacterial clone)

Prone to batch to batch variability

Their homogeneous nature ensures better reproducibility between tests, where conditions are kept constant. Useful for quantification experiments like flow cytometry

Their heterogeneity means they are more tolerant to slight changes to the antigen e.g. denaturation, polymorphism or differences in glycosylation state

Because of their specificity, they are less likely to cross-react with other proteins, giving lower background than polyclonal immunoglobulins

May contain non-specific immonoglobulin resulting in background staining

Specificity makes them ideal as the primary immunoglobulin in an assay, for detecting antigens in tissue, or for affinity purification of antigens

Useful as secondary immunoglobulins or for immunoprecipitation, as they target multiple epitopes providing a more robust detection


Immonoglobulin Structure

The classical description of an immunoglobulin is “Y” shaped consisting of four polypeptide subunits – two identical heavy chains and two identical light chains (see Figure 1). The N-terminus of each heavy chain associates with one of the light chains to create two antigen-binding domains; these are the arms of the “Y” shape and are termed Fragment Antigen Binding (Fab) fragments or domains.

The C-termini of the two heavy chains combine to form the Fc domain (fragment crystallization); this is the tail of the “Y”. The Fc domain is important for the immunoglobulin’s interaction with effector cells such as macrophages and also the activation of the complement cascade. The four polypeptide chains are held together by covalent disulfide bridges and non-covalent bonds.

Immunoglobulin structure, antibody structure

 

Figure 1. Structure of an immunoglobulin

Sequence analysis of various light chains has revealed two distinct regions (see Figure 2) – a highly variable N-terminal half (VL) and a constant C-terminal half (CL). Similar studies on heavy chains have shown they also contain variable and constant regions (VH and CH, respectively).

However, heavy chains contain one variable region (found at the N-terminus) and three constant regions (see Figure 2). Unsurprisingly, the variable regions of both chains localize to the Fab fragments, and this provides the structural basis for the antigen and epitope-selectivity of immunoglobulins.

The sequence heterogeneity within the variable regions is not random, but occurs at three short (5-30 amino acid) segments of each chain called the ‘‘hypervariable regions’’. As the residues in hypervariable regions form the actual binding site for the antigen, they are also known as the complementarity determining regions or CDRs (see Figure 2).

Immunoglobulin structure - light and heavy chain structure

Figure 2. Light and heavy chain structure. Light chains are approximately 220 amino acids in length and can be divided into two equal 'variable' and 'constant' regions. Heavy chains are about 440 amino acids in length and are subdivided into four 110 amino acid segments - one variable region and three constant regions.


Immunoglobulin Manufacturing Tools

Immunoglobulin Manufacturing Tools

Want to make your own immunoglobulin? As an antibody manufacturer with over 30 years of experience, AbD Serotec offers assistance with immunoglobulin production, conjugation of antibodies and for the generation of new monoclonal immonuglobulin in your laboratory or ours.

Some of our immunoglobulin manufacturing tools include:


AbD Serotec

AbD Serotec is a global antibody manufacturer and supplier of over 11,000 immunoglobulins, kits and accessories.

We provide great customer support for all of our products and services so please contact us with any questions on their use or the results you can expect. We can even help source products not currently available or develop them to your specification using our custom immunoglobulin services.


References

(1) Köhler and Milstein, C. (1975) Continuous cultures of fused cells secreting antibody of predefined specificity. Nature 256: 495-497


Author: Misha Rahman, Ph.D.