SPA can bind to Fc fragments in serum IgG molecules of human beings and many mammals, and the binding affinity order is pig, dog, rabbit, human, monkey, mouse, mouse and cow. Poor affinity for rats and sheep; Have no affinity for horses, calves, goats, etc. Can not be combined with all fish, amphibians, reptiles and birds (with a few exceptions). Chicken IgG must form a complex with the corresponding antigen to bind to SPA, which may be because the Ag-AB complex changes the conformation of the Fc fragment of IgG.

SPA-bound IgG subclasses mainly include IgG 1, IgG2 and IgG4.

In addition to IgG, SPA can also bind a small amount of IgM and IgA in serum. The adsorption rates of various immunoglobulins by SPA bacteria are 90% ~ 98%, 2% ~ 30% for IgM and 1.5% ~ 20% for IgA.

SPA and IgG with sedimentation reaction must have two binding sites, one on Fc and the other on Fab fragment. Although one of them can bind to SPA, there is no precipitation reaction, and the participation of Fab is not a SPA- antibody reaction. Studies show that the binding site between IgG and SPA is the binding site of CH2 and CH3.