1. Engulfed bacteria inside a phagosome
or a phagolysosome.
2. An activated TH1 lymphocyte binds to a peptide/MHC-II complex on a macrophage by way of its TCR and CD4 molecule. Co-stimulatory molecules such as CD40L on the TH1 cell then bind to CD40 on a macrophage.
3. This triggers the TH1 lymphocyte to secrete the cytokine interferon-gamma (IFN-γ) that binds to IFN-γ receptors receptors on the macrophage.
4. The IFN-γ activates the macrophage enabling it to produce more hydrolytic lysosomal enzymes, nitric oxide, and toxic oxygen radicals that destroy the microorganisms within the phagosomes and phagolysosomes.