Which of the following is considered a sign of successful infection fighting by white blood cells?

The presence of pus at the site of infection is a strong indicator of the immune system actively fighting off an infection. Pus is typically composed of dead white blood cells, bacteria, and tissue debris. When an infection occurs, white blood cells, particularly neutrophils, migrate to the site to combat the invading pathogens. As these cells work to eliminate the infection, they often accumulate in large numbers, along with their byproducts, leading to the formation of pus. This is a visible sign that the immune response is in action and that the body is attempting to clear the infection.

Other signs, such as increased body temperature, improved appetite, or reduced inflammation, can indicate changes in the body's response but do not specifically signify that the fighting process through the influx of immune cells is actively occurring at the site of infection. Increased body temperature may indicate a fever, which is part of the immune response but not a direct sign of white blood cells fighting; it is a systemic response. Improved appetite could suggest recovery but does not indicate the presence of immune activity, while reduced inflammation indicates healing rather than active fighting against an infection. Thus, the presence of pus distinctly shows that the immune response is working at that localized area.