A healthy individual has about 4,500 to 11,000 leukocytes or white blood cells per microliter of blood, accounting for about 1% of blood volume. Unlike erythrocytes, this group of cells lacks hemoglobin but is considered complete as they possess a nucleus and other cellular organelles. Most leukocytes reside in the connective tissues, mainly in the lymphatic organs like the spleen and lymph nodes, with a small fraction in circulating blood. As part of the immune system, leukocytes protect the body from threats like pathogens, toxins, and abnormal body cells. In case of an infection or injury, the circulating leukocytes are activated, and their numbers double up in a few hours, a process called leukocytosis. The damaged or infected tissues and circulating pathogens release chemical stimuli, attracting activated leukocytes in the blood. Some leukocytes are capable of phagocytosis. These leukocytes can also adhere to and react with endothelial cells lining the blood vessel, squeezing out to enter the tissue. As a result, they can clear the infected tissue cells and their debris along with circulating pathogens.