Part 3/14:
Once pathogens breach physical barriers, the innate immune system takes over. This rapid response involves white blood cells, including neutrophils, macrophages, and natural killer cells, which migrate swiftly to infection sites. These cells synergize with complement proteins—circulating blood components that "tag" bacteria or viruses for destruction and stimulate immune cells to attack invaders. Cytokines, signaling molecules such as interleukins and tumor necrosis factors, amplify this response by alerting and coordinating immune activity.