Immunoglobulin D (IgD) is important in the early stages of the immune response. Unlike other antibodies, it does not actively circulate but instead binds to B cells to instigate the immune response.

There are five main types (isotypes) of antibodies, each with distinct structures and functions: IgG, IgM, IgA, IgE, and IgD. Illustration of the five major antibody (immunoglobulin) types ...