## Intracellular Habitat of Leishmania donovani **Key Point:** Leishmania donovani amastigotes survive and multiply within macrophages and cells of the reticuloendothelial system (RES), making these cells the primary site of parasitic residence in kala-azar. ### Cellular Tropism and Pathology | Cell Type | Role in Kala-azar | Outcome | |-----------|-------------------|----------| | Macrophages | Primary intracellular habitat | Parasites multiply; cell activation impaired | | Splenic macrophages | Major site of parasite burden | Massive splenomegaly | | Hepatic macrophages (Kupffer cells) | Secondary site | Hepatomegaly | | Bone marrow macrophages | Parasite reservoir | Pancytopenia development | | Lymph node macrophages | Early infection site | Lymphadenopathy | ### Mechanism of Intracellular Survival 1. Promastigotes injected by sandfly are phagocytosed by macrophages 2. Transform to amastigotes within phagolysosome 3. Amastigotes resist lysosomal enzymes via surface lipophosphoglycan (LPG) 4. Multiply by binary fission within macrophage cytoplasm 5. Macrophage eventually ruptures, releasing parasites to infect more cells **High-Yield:** The massive splenomegaly in kala-azar (up to 2–3 kg) is due to accumulation of parasitized macrophages and reactive lymphocytes, not direct parasitization of splenic tissue. **Mnemonic:** **RES = Reticuloendothelial System** — Remember that Leishmania targets the RES organs: **Spleen, Liver, Bone marrow, Lymph nodes**. **Clinical Pearl:** The inability of infected macrophages to mount an effective immune response (due to IL-10 and TGF-β production) allows unchecked parasite multiplication, leading to the characteristic pancytopenia and immunosuppression seen in advanced kala-azar.
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