As outlined in the previous chapter, red blood cells (RBC) in static human blood form loose aggregates with a characteristic face-to-face morphology, similar to a stack of coins, termed rouleaux. Such aggregation is caused by the presence of various macromolecules, such as fibrinogen, in the plasma. Similar aggregation can also be caused if RBC are suspended in solutions of high molecular mass water-soluble polymers such as dextran, poly(ethylene glycol) abbreviated PEG, or polyvinylpyrrolidone (PVP). However, RBC aggregation does not occur in simple salt solutions (e.g., phosphate-buffered saline, PBS). Figure 2.1 shows human RBC from the same donor suspended at a low hematocrit in PBS (panel a) and in autologous plasma (panel b). RBC suspended in plasma form rouleaux, while no aggregation occurs for RBC in PBS, thus demonstrating the important role of plasma factors.