Immunoglobulins (Ig)
Glycoprotein molecules that are produced by plasma cells in response to an immunogen and which function as antibodies. The immunoglobulins derive their name from the finding that they migrate with globular proteins when antibody-containing serum is placed in an electrical field  (Figure 1).

tion by Design  © 2000, 

  Figure  6   Immunoglobulin fragments: Structure/function relationships

VI. HUMAN IMMUNOGLOBULIN CLASSES, SUBCLASSES, TYPES AND SUBTYPES

A. Immunoglobulin classes
The immunoglobulins can be divided into five different classes, based on differences in the amino acid sequences in the constant region of the heavy chains. All immunoglobulins within a given class will have very similar heavy chain constant regions. These differences can be detected by sequence studies or more commonly by serological means (i.e. by the use of antibodies directed to these differences).

1. IgG - Gamma  heavy chains

2. IgM - Mu  heavy chains

3. IgA - Alpha heavy chains

4. IgD - Delta  heavy chains

5. IgE - Epsilon  heavy chains

B. Immunoglobulin Subclasses
The classes of immunoglobulins can de divided into subclasses based on small differences in the amino acid sequences in the constant region of the heavy chains. All immunoglobulins within a subclass will have very similar heavy chain constant region amino acid sequences. Again these differences are most commonly detected by serological means.

1. IgG Subclasses

a) IgG1 - Gamma 1  heavy chains

b) IgG2 - Gamma 2  heavy chains

c) IgG3 - Gamma 3  heavy chains

d) IgG4 - Gamma 4  heavy chains

2. IgA Subclasses

a) IgA1 - Alpha 1  heavy chains

b) IgA2 - Alpha 2  heavy chains

C. Immunoglobulin Types
Immunoglobulins can also be classified by the type of light chain that they have. Light chain types are based on differences in the amino acid sequence in the constant region of the light chain. These differences are detected by serological means.

1. Kappa light chains 

2. Lambda light chains 

D. Immunoglobulin Subtypes
The light chains can also be divided into subtypes based on differences in the amino acid sequences in the constant region of the light chain.

1. Lambda subtypes

a) Lambda 1 

b) Lambda 2 

c) Lambda 3 

d) Lambda 4 

E. Nomenclature
Immunoglobulins are named based on the class, or subclass of the heavy chain and type or subtype of light chain. Unless it is stated precisely you are to assume that all subclass, types and subtypes are present. IgG means that all subclasses and types are present.

F. Heterogeneity
Immunoglobulins considered as a population of molecules are normally very heterogeneous because they are composed of different classes and subclasses each of which has different types and subtypes of light chains. In addition, different immunoglobulin molecules can have different antigen binding properties because of different V_H and V_L regions.