The zeta potential (ZP) theory is a theory that partially explains why red cells suspended in saline repel each other, and thus make agglutination by small IgG molecules difficult.
The surface of red cells carry a negative charge due to the ionization of the carboxyl group of NeuNac (N-acetyl neuraminic acid), also called NANA or sialic acid. In saline, red cells will attract positively charged Na+, and an ionic cloud will form around each cell. Thus the cells will be repelled and stay a certain distance apart. Zeta potential is a measure of this repulsion and is measured in microvolts at the boundary of sheer or slipping plane. The ZP is a measure of the difference in charge at the surface of the membrane and the outer edge of the ionic cloud (slipping plane). For IgG molecules to span the distance between red cells in saline, the ZP must be reduced so the cells can come closer. (Schematic of zeta potential theory.)