Final answer:
Interferon-gamma (IFN-γ) secreted by Th1 cells is the primary cytokine that induces MHC Class II expression in macrophages, crucial for antigen presentation and T cell activation in the immune response.
Step-by-step explanation:
The cytokine primarily responsible for inducing MHC Class II expression on macrophages is Interferon-gamma (IFN-γ), which is secreted by Th1 cells. IFN-γ plays a critical role in the immune response by activating macrophages, enhancing their ability to process and present antigens via MHC Class II molecules to CD4+ helper T cells. The interaction between MHC Class II on macrophages and CD4 molecules on T cells is essential for the adaptive immune response, allowing for the activation and proliferation of T cells that will ultimately help clear pathogens.
Professional antigen-presenting cells, such as macrophages, dendritic cells, and B cells, are the only cell types that express MHC Class II on their surface. Another important cytokine involved in the immune response is Interleukin-4 (IL-4), which can also influence the expression of MHC Class II on certain immune cells.