Do plasma proteins adsorb to red cells?

The weak aggregation of red cells, referred to as rouleaux formation, has been explained by the non-specific adsorption of plasma proteins to these cells in a bridging configuration. However, no unequivocal measurement of sufficient protein adsorption to cause aggregation has been reported. Aggregation induced by a depletion mechanism which requires no protein adsorption is also possible. No definitive information is currently available which proves either model but we find little support for the bridging hypothesis in either our own work or in that of others.