Evasion mechanisms of protozoa fall into three major categories. The first is entrance of the organism into the host cell, where it avoids immune surveillance. One example is malaria, as noted previously. Others include toxoplasma, leishmania, and Trypanosoma cruzy, which enter and can grow inside macrophages. For example, leismania binds C3 avidly and thus serves as a ligand for the CR3 receptor on macrophages. If one uses monoclonal antibodies to the CR3 receptor, this inhibits the uptake of the parasite into macrophage. Another approach is used by toxoplasma in which they prevent fusion of phagocytic vacuoles containing the paracite with lysosomes and thus are not destroyed. Finally, trypanosomes need activated macrophages for killing; thus they are resistant to intracellular killing in nonactivated macrophages.