Which of the following chemicals is released by one type of immune cell to directly activate another immune cell?

Lysozymes are enzymes that break down bacterial cell walls, providing a defense mechanism against pathogens. While they play a role in the immune response, they do not function as signaling molecules to activate other immune cells. Instead, they are primarily involved in the innate immune response and are not directly linked to intercellular communication among immune cells.

Perforin is a protein released by cytotoxic T cells and natural killer cells that forms pores in the membranes of target cells, leading to their destruction. While perforin is important for killing infected or malignant cells, it does not serve to activate other immune cells; rather, it functions in the direct elimination of threats.

Granzymes are serine proteases released alongside perforin by cytotoxic T cells to induce apoptosis (programmed cell death) in target cells. Similar to perforin, granzymes are involved in the destruction of infected or cancerous cells rather than in the activation of other immune cells. They do not play a role in intercellular signaling.