Immunoglobulins (also known as antibodies) are proteins produced by plasma cells in the immune system that help the body fight infections. They bind to harmful invaders like bacteria and viruses, marking them for destruction by other immune cells. There are five main types of immunoglobulins: IgG, IgA, IgM, IgD, and IgE, each serving a different function in immune response.

In multiple myeloma, abnormal plasma cells produce excessive amounts of a single type of immunoglobulin, often referred to as monoclonal protein (M protein). This overproduction of a specific immunoglobulin disrupts the normal balance and function of the immune system, leading to various complications.