Red blood cells are known to form aggregates in the form of rouleaux. This
aggregation process is believed to be reversible, but there is still no full
understanding on the binding mechanism. There are at least two competing
models, based either on bridging or on depletion. We review recent experimental
results on the single cell level and theoretical analyses of the depletion
model and of the influence of the cell shape on the binding strength. Another
important aggregation mechanism is caused by activation of platelets. This
leads to clot formation which is life saving in the case of wound healing but
also a major cause of death in the case of a thrombus induced stroke. We review
historical and recent results on the participation of red blood cells in clot
formation
