Protein A is a Staphylococcus aureus cell wall constituent that is characterized by its binding affinity to the Fc portion of some immunoglobulins, especially those of the IgG class. Because of this property, it is often utilized for microscopic visualization of molecules and to purify antisera. Protein A has also been used for the treatment of proteinuria in nephrotic syndrome, and severe autoimmune diseases such as rheumatoid arthritis, coeliac disease, and systemic lupus erythematosis.
