An immunoglobulin (antibody) with δ-type of heavy chain, occurs primarily as an antibody (along with IgM) to be expressed on the surface of B cells, and therefore, is thought to be associated with B cell development and activation
The plasma cell produces immunoglobulins (or antibodies) as an immune response to an antigen, i.e. non-self agent. An immunoglobulin (Ig) is a glycoprotein with a Y-shape. The basic structure of a monomeric unit of antibody consists of two identical heavy chains and two identical light chains. In each of these chains, there are variable and constant regions. The immunoglobulin heavy chain, in particular, is the large polypeptide subunit. It can be used to classify the various isotypes (or classes) of immunoglobulins. In humans and other mammals, the five classes are: (1) immunoglobulin A (IgA), (2) immunoglobulin D (IgD), (3) immunoglobulin E (IgE), (4) immunoglobulin G (IgG), and (5) immunoglobulin M (IgM).
The immunoglobulin D (IgD) has δ (gamma)-type of immunoglobulin heavy chain (thus, the acronym D). The light chain of IgD may either be kappa (κ) or lambda (λ). Its molecular formula is δ2 κ2) or δ2λ2. Similar to common immunoglobulins, IgDs are produced and expressed on the surface of plasma cells. Some of the IgDs are released into the bloodstream. However, the amount of IgDs in serum is very low (i.e. 0.2% of the total immunoglobulins). In their secreted form, IgDs are monomers. That means they have only one functional unit of antibody.
The immune function of IgDs is still unclear. Nevertheless, they are thought to be associated with B cell activation.