Immunoglobulin structure

Immunoglobulins (Ig) are composed of two polypeptide chains known as heavy chains and two polypeptide chains known as light chains that are linked together by chemical bonds (Figure 5). There are five different classes of Ig, each with unique variations in their structure.

Figure 5. Structure of an immunoglobulin including two heavy chains and two light chains

The Ig classes are defined by the nature of the heavy chain (Table 1).

There are five different types of heavy chain proteins termed G, A, D, E and M and two types of light chain proteins termed kappa (κ) and lambda (λ).

Each Ig is made up of two of any one type of heavy chain and two of one of the light chain types and is classed depending on which heavy chain it possesses. Therefore Igs are referred to as either: IgG, IgA, IgD, IgE or IgM. Each Ig can be further sub-classed depending on the type of light chain it has. For example, an Ig with ‘G’ heavy chains and lambda light chains would be referred to as IgG lambda.

Healthy individuals produce a mixture of the different Igs (known as polyclonal immunoglobulins). Each plays a specialised role in fighting infection.

Myeloma cells, however, only produce one type of immunoglobulin (monoclonal immunoglobulin). This monoclonal immunoglobulin is commonly referred to as paraprotein or M-protein: it has no useful function and cannot help fight infection.

Table 1: Immunoglobulin classes and functions