Immunoglobulin, also known as antibodies, are proteins produced by the immune system to identify and neutralize foreign objects like bacteria and viruses. There are five main classes of immunoglobulins: IgG, IgA, IgM, IgE, and IgD, each serving different roles in immune response. These proteins bind to specific antigens, which are substances that trigger an immune response. By doing so, immunoglobulins help mark these invaders for destruction by other immune cells, playing a crucial role in protecting the body from infections and diseases.