Final answer:
White blood cells are a critical part of the immune system, with various types performing different defensive roles against pathogens. They are grouped into granulocytes and agranulocytes, and work by patrolling the body for internal and external threats.
Step-by-step explanation:
White blood cells (WBCs), also known as leukocytes, play a crucial role in the immune system by defending the body against both internal and external threats like pathogens, including bacteria, viruses, and other foreign invaders. There are different types of white blood cells, such as neutrophils, lymphocytes, monocytes, eosinophils, and basophils, which carry out various functions including phagocytosis, immune surveillance, and the release of histamines and other inflammation mediators. White blood cells can be categorized into two main groups: granulocytes, which contain visible granules in the cytoplasm, and agranulocytes, which do not.
These cells are produced in the bone marrow and circulate in the blood and lymphatic system. Some white blood cells, like macrophages derived from monocytes, can migrate from the bloodstream to infected tissues to engulf and digest pathogens in a process called phagocytosis. Others, such as lymphocytes, play a key role in the adaptive immune response by identifying specific antigens and either directly attacking infected cells or creating antibodies.