Leukocytes, also known as white blood cells, are key parts of the immune system. They move throughout the body to find and remove different pathogens. The immune system uses three main lines of defense. The first line includes physical barriers like the skin. The second line involves general, non-specific responses inside the body. The third line is adaptive immunity, which remembers past infections. In the second line, neutrophils act as first responders and eat harmful invaders. Eosinophils fight parasites and help control allergies. Basophils and mast cells release histamine, which causes inflammation. Macrophages and dendritic cells clean up debris and help connect the innate and adaptive systems. Natural Killer (NK) cells are also important in this stage because they destroy virus-infected or cancerous cells without needing to recognize them first. The third line of defense uses B cells to make specific antibodies and T cells to organize the immune response or kill infected cells. This helps protect the body from future infections.