These cells fight viral, bacterial, and other infections and participate in the hypersensitivity responses seen in allergic reactions. Neutrophils, basophils, and eosinophils are granulocytes - white blood cells whose main purpose is to destroy bacteria. Granulocytes are distinguished by the small particles, or granules, that reside within each cell and contain substances to fight infections. Granulocytes undergo many stages of development before becoming mature neutrophils, basophils, or eosinophils. They contain cellular systems that consume foreign substances by surrounding and digesting of microorganisms and foreign particles or enveloping foreign substances within the plasma membrane.
Monocytes originate in the bone marrow as immature monoblasts and promonocytes.
Once these early cell forms develop into monocytes, they circulate in the bloodstream for about 24 hours. If the monocytes detect an area of inflammation, they move into tissues to become macrophages - larger phagocytic cells that help lymphocytes to detect foreign microorganisms and launch immune system responses. Lymphatic tissue contains both fixed and circulating elements. Many different types of lymphatic cells interact to combat infections and recognize abnormal cells within the body. Fixed lymphoid tissue is found in the lymph nodes, spleen, thymus, tonsils and adenoids, bone marrow, and various sites within the gastrointestinal tract, respiratory system, and liver.