Infected erythrocytes (red blood cells) tend to have lower oxygen tension, because it is significantly reduced by the parasite. This causes sickling of that particular erythrocyte, signalling the phagocytes to get rid of the cell and hence the parasite within. Since the sickling of parasite-infected cells is higher, these selectively get removed by the reticulo-endothelial system, thus sparing the normal erythrocytes.