Proteins in the blood also known as antibodies. There are 5 major types of antibodies including IgG, IgA, IgM, IgE, and IgD. Each antibody type has a specific and unique function in the immune system. IgG is the most abundant type in blood and is important in fighting certain types of infections. IgG can be purified from blood to make immunoglobulin products that can be transfused into others who are deficient.