Immunoglobulin D (IgD) is one of the five main classes of antibodies found in the human body. It plays a crucial role in the immune system, primarily functioning as a receptor on the surface of B cells, which are essential for producing antibodies. IgD helps in the activation and regulation of these B cells, contributing to the body's ability to respond to infections. Although IgD is present in small amounts in the bloodstream, its exact function is not fully understood. It is believed to be involved in the initiation of immune responses and may help in recognizing specific pathogens. Overall, IgD is an important component of the immune system, aiding in the body's defense mechanisms.