How does Leishmania infect macrophages in the immune system?

Leishmania parasites infect macrophages through a cycle involving two distinct morphological stages. The infection is initiated when an infected sand fly bites a vertebrate host, introducing the motile, flagellated form of the parasite—known as a promastigote or leptomonad—into the wound.12

Once inside the vertebrate host, these parasites invade macrophages, which are immune cells that typically function to engulf and destroy foreign pathogens. Within the macrophage, the parasite transforms into a round, nonmotile form called an amastigote. In this intracellular state, the amastigotes are able to survive, grow, and multiply.132

Depending on the specific species of Leishmania and the host's immune response, the infected macrophages may remain localized at the site of the bite, causing cutaneous lesions, or they may be transported through the blood to visceral organs such as the liver, spleen, and bone marrow, leading to systemic leishmaniasis.13