Protein A:

• Is a surface protein originally found in the cell wall of Staphylococcus aureus
• Has a molecular size of 42kDa
• Is expressed by Staphylococcus aureus
• Does not have binding sites for serum albumin
• Can be used to purify human antibodies such as IgA, IgM, and IgE because it has a higher ability to bind to human antibodies other than IgG antibody
• Cannot be used for purification of the human IgG3 subclass as it cannot bind to the human IgG3 subclass immunoglobulin.

Protein G:

• Is a surface immunoglobulin binding protein specifically expressed by group C and D streptococcal bacteria
• Has a molecular size of approximately 65kDa
• Is expressed by group C and D streptococcal bacteria
• Has binding sites for serum albumin
• Can be used to purify the human IgG3 subclass as it is binding to the human IgG3 subclass immunoglobulin
• Can bind to all subtypes of human IgG antibodies but not to other human antibodies such as IgA, IgE, and IgM 
• Cannot be used to purify these human antibodies IgA, IgE and IgM