Antibody types and structure
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The structural components of an antibody
Antibodies are a type of glycoproteins, which bind to specific antigens and are synthesised by B-cells as a response to foreign molecule invasion in the body, and are made from four polypeptide chains, creating the well-known Y-unit.
Furthermore, each Y section consists of an identical pair of heavy chains and an identical pair of light chains, named after their length and sequence differences and connected by disulfide bonds. What differentiates one antibody from another is its variable region, located at the top of the Y section. This region is responsible for the tight and specific binding to the epitope of the antigen, that the immunoglobulin has been secreted against.
Light and heavy chains
The biggest difference between the two types of chains is the number of constant domains that they contain. In the case of light chains, they have one variable, and one constant domain and heavy chains have one variable and either three or four constant domains. The light chains can be further categorised into two classes: kappa and lambda depending on small polypeptide sequence differences.
On the other hand, the heavy chain components are responsible for differentiating between the different immunoglobulin classes, and the five types of heavy chains are denoted with the letters α, δ, ε, γ and μ, with each chain found in the isotype with the same name. These chains consist of a constant and variable region with a hinge region which aids in flexibility.
Isotypes
The five isotypes that can be found in mammals are IgG, IgA, IgD, IgE and IgM. The categories are based on the type of heavy chain and number of Y units present, which contribute to the different properties and locations that each category of antibodies possess.
IgA
This type of immunoglobulins is the most produced by the B-cells and are found in a variety of mucosal areas, including the respiratory and digestion tract, and the gut. Their main function is to prevent pathogen colonisation of the mucosal areas and because of their localisation in the body, they are resistant to digestion and can also be found in milk, saliva and tears.
IgD
This antibody isotype mainly acts as an antigen receptor on B-cells that are yet to be exposed to antigens and is related to basophil and mast cell activation in order to produce antimicrobial factors.
IgE
These immunoglobulins came into existence when humans and mammals evolved in order to defend themselves against parasitic worms but in present times their primary function is to bind allergens and to trigger histamine release from basophils and mast cells.
IgG
These immunoglobulins exist in four different forms from IgG1 to IgG4 and together they are responsible for most of the antibody immunity against pathogen invasion. Furthermore, IgG antibodies are the only type that can cross the placenta and give the fetus passive immunity.
IgM
IgM type antibodies are found on the B-cell surface in their monomer form or they can be secreted as a pentamer with exceptionally high avidity. Their main role is to fight off pathogens in the first stages of humoral immunity while the number of secreted IgG is sufficient to continue the pathogen elimination.
F(ab) and Fc regions
The distinctive Y-shape of immunoglobulins can be separated into two F(ab) regions and one Fc region, with the first containing the variable domain responsible for cognate antigen binding. The responsibilities of the Fc fragment are to provide a binding site for endogenous Fc receptors on the lymphocyte surface and it also serves as the binding site for secondary antibodies. Furthermore, dye and enzyme conjugation is possible because of this region.
To extend the versatility of antibodies in different immunochemical procedures, the proteolytic enzyme pepsin can be used to cleave the three regions. This separation is useful in radiolabelling, immunohistochemistry and live studies, as each of the two types of fragments has its unique characteristics that sometimes cannot be exhibited unless they have been cleaved from the other region and used on their own.