As outlined in the previous chapter, there is general agreement regarding correlations between elevated levels of fibrinogen or other large plasma proteins and enhanced red blood cell (RBC) aggregation, as well as the effects of molecular mass and concentration for neutral polymers such as dextran or poly(ethylene glycol) (PEG) (Chien and Lang 1987). It has also been shown that cellular factors affect RBC aggregation via changing the intrinsic tendency of RBC to form aggregates (i.e., RBC aggregability, see Chapters 1 and 2) (Rampling et al. 2004), and that specific cellular factors can be linked to the observed changes in aggregability.