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B cells form billions of antibodies, and antigen-binding glycoproteins collectively called immunoglobulin (Igs). Each immunoglobulin contains a specific amino acid sequence and different antigen-binding sites.

Antibodies are bivalent and show Y-shaped structures. The Y shape has two identical antigen-binding sites. They are made up of four polypeptide chains. Antibodies are heterodimers and show the presence of light chains and heavy chains.

The non-particulate substances such as the macromolecules of antigens are engulfed by the energy-dependent process called endocytosis.

Macrophages possess receptors that recognize non-particulate substances or foreign material. Activation of certain macrophages increases the process of endocytosis.

The small part of the antigen which binds with antibodies is called the antigenic determinant or an epitope. In the diagram, the pencil-shaped protein has two epitopes or valence. It means two antibodies can attach it at a time.

The binding of an epitope of an antigen with the antibodies triggers the immune response.