Antibody Structure and Classes

Antibodies, also known as immunoglobulins, are soluble proteins produced by effector B cells called plasma cells in response to foreign substances like bacteria and viruses.

The primary antibody monomer consists of two identical heavy and light chains held together by disulfide bonds, forming a Y-shaped structure.

At the two tips of the Y arms, the heavy and light chains form the variable region with antigen-binding sites.

These sites are unique for each antibody molecule, making them specific in their antigen interactions.

The stem of the Y-shape has the constant region predominated by the heavy chains, which determines the antibody class independent of the antigen.

Based on the constant region, antibodies are designated into five classes —  IgM, IgD, IgG, IgA, and IgE.

Each class has distinct characteristics and structures that serve specific biological functions.

For example, antibodies from class IgM typically exist as pentamers of five Y-shaped units with ten antigen-binding sites, while IgA antibodies can form dimers.

IgM is the first responder during an infection, and IgA is present in secretions like saliva.