The immune system protects the body from harmful pathogens like bacteria and viruses while keeping the body balanced. It works through three lines of defense. The first line includes barriers like skin and mucus that prevent pathogens from entering. The second line is the innate immune response, where cells attack invaders quickly and non-specifically. The third line is the adaptive immune response, which targets specific pathogens and creates memory for future protection. Leukocytes, or white blood cells, play a key role and include nine types such as neutrophils, macrophages, natural killer cells, B cells, and T cells. Each type helps by attacking pathogens, producing antibodies, or coordinating the immune response.