Protein A and G Specificities
| Species | Immunoglobulin | Protein A binding | Protein G binding |
| Human | IgA | + | - |
| IgA1 | + | - | |
| IgA2 | + | - | |
| IgD | - | - | |
| IgE | ++ | - | |
| IgG | ++++ | ++++ | |
| IgG1 | ++++ | ++++ | |
| IgG2 | ++++ | ++++ | |
| IgG3 | - | ++++ | |
| IgG4 | ++++ | ++++ | |
| IgM | - | - | |
| Mouse | IgG1 | ++++ | ++++ |
| IgG2a | ++++ | ++++ | |
| IgG2b | +++ | +++ | |
| IgG3 | + | +++ | |
| IgM | - | - | |
| Rat | IgG1 | - | + |
| IgG2a | - | ++++ | |
| IgG2b | - | ++ | |
| IgG2c | + | ++ | |
| Bovine (Cow) | IgG | ++ | ++++ |
| Feline (Cat) | IgG | ++++ | - |
| Chicken | IgG | - | + |
| Canine (Dog) | IgG | ++++ | ++++ |
| Caprine (Goat) | IgG | - | ++ |
| Guinea Pig | IgG | ++++ | ++ |
| Hamster | IgG | + | ++ |
| Equine (Horse) | IgG | ++ | ++++ |
| Porcine (pig) | IgG | +++ | +++ |
| Monkey | IgG | +++ | +++ |
| Rabbit | IgG | ++++ | +++ |
| Ovine (Sheep) | IgG | +/- | ++ |
Protein A is a 42 kDa protein isolated from Staphylococcus aureus by Cohen and Sweeney. It binds to the Fc region of IgG without affecting antigen binding. It suffers, in addition to varable binding depending on IgG subtype, from albumin binding. Protein G is a modified protein which does not bind albumin and binds more IgG types.