IgD is a type of antibody, also known as immunoglobulin D, that plays a role in the immune system. It is primarily found on the surface of B cells, which are a type of white blood cell. IgD helps in the activation and regulation of these B cells, contributing to the body's ability to respond to infections. Unlike other antibodies, IgD is present in very small amounts in the bloodstream. Its exact function is not fully understood, but it is believed to be involved in the initiation of immune responses and may help in recognizing specific pathogens.