Activation of which cells is important for defense in Leishmania pathogenesis?

• A. TH1 cells
• B. TH2 cells
• C. B cells
• D. NK cells

Answer: (A)

Defense against Leishmania relies on cell-mediated immunity because these parasites live inside macrophages. When TH1 cells are activated, they produce interferon-gamma, which tells macrophages to ramp up killing mechanisms—nitric oxide and reactive oxygen species—within the infected cells. This macrophage activation is what enables the intracellular Leishmania to be controlled and eliminated, preventing spread and disease.

In contrast, a TH2-skewed response tends to drive antibody production and can promote macrophage states that are less effective at killing intracellular pathogens, which can lead to poorer control of the infection. NK cells can contribute early by releasing IFN-γ, but the sustained, protective effect that clears intracellular Leishmania mainly comes from TH1-driven activation of macrophages.

So, activating TH1 cells is the best answer because their IFN-γ–mediated signaling directly enhances macrophage killing of parasites hiding inside those cells.