Final answer:
Pus accumulates when leukocytes, especially neutrophils, destroy bacteria. Neutrophils release toxins to combat infection and the remnants from this process, along with dead bacteria and tissue fluid, form pus.
Step-by-step explanation:
A substance that often accumulates when leukocytes are actively destroying bacteria is pus. This accumulation happens because neutrophils, a type of phagocytic leukocyte, are rapid responders at the site of infection. They engulf bacteria and, once they've done their job and die, their remains contribute to pus formation. Pus is composed of dead pathogens, neutrophils, tissue fluid, and other cells destroyed during the immune response. Neutrophils release toxins like lysozyme, hydrogen peroxide, and defensins that kill or inhibit bacteria and fungi, and they also recruit other immune cells to combat infection.