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. Immunoglobulins work by binding to specific antigens, which are substances that trigger an immune response. This binding helps to mark the antigens for destruction by other immune cells, facilitating the body's defense against infections and diseases.