Leukocytes, or white blood cells, are diverse cells forming a core part of the body’s immune system. They circulate throughout the bloodstream and tissues, defending against foreign invaders like bacteria, viruses, fungi, and abnormal cells. Leukocytes are produced in the bone marrow and stored in various parts of the body, including blood and lymphatic tissues.
Granulocyte Characteristics and Roles
Granulocytes are leukocytes characterized by visible granules in their cytoplasm, containing enzymes for defense. These cells have a lobed nucleus. There are three primary types of granulocytes, each with specialized roles in the immune response.
Neutrophils are the most abundant granulocytes, making up 50-70% of all white blood cells. They are first responders to infections, especially bacterial and fungal invasions, rapidly migrating to inflammation sites. Neutrophils combat pathogens through phagocytosis, engulfing and digesting harmful microorganisms. Their activity and death contribute to pus formation at infection sites.
Eosinophils account for 1-3% of circulating leukocytes, involved in allergic reactions and combating parasitic infections. These cells have a bilobed nucleus and their granules stain reddish-orange. Eosinophils release proteins toxic to parasites. They also modulate immune responses by releasing cytokines.
Basophils are the least common granulocytes, making up less than 1% of white blood cells. These cells have a two-lobed nucleus and contain large granules that stain dark blue or purple. Basophils are involved in allergic and inflammatory responses, releasing chemicals like histamine and heparin. Histamine widens blood vessels and increases blood flow to injured tissues, while heparin acts as an anticoagulant, preventing clotting and promoting white blood cell movement.
Agranulocyte Characteristics and Roles
Agranulocytes are leukocytes distinguished by the absence of prominent granules in their cytoplasm and a large, unlobed nucleus. The two main types are lymphocytes and monocytes, both playing distinct roles in the body’s defense system.
Lymphocytes are central to adaptive immunity, providing specific, long-lasting protection against pathogens. They comprise 20-40% of circulating leukocytes. Lymphocytes categorize into T cells, B cells, and Natural Killer (NK) cells. T cells attack infected or cancerous cells, while B cells produce antibodies targeting specific pathogens. NK cells, part of the innate immune system, destroy abnormal cells like those infected with viruses or cancerous cells through non-specific mechanisms.
Monocytes are the largest leukocytes, characterized by their large, kidney-shaped or bean-shaped nucleus and clear cytoplasm. They circulate in the bloodstream before migrating into tissues, differentiating into macrophages or dendritic cells. Macrophages are powerful phagocytes that engulf and digest pathogens, cellular debris, and apoptotic cells. They also present antigens, displaying pathogen fragments to lymphocytes to activate adaptive immune responses. Dendritic cells, derived from monocytes, act as sentinels in tissues, capturing antigens and presenting them to lymphocytes in lymph nodes, initiating specific immune responses.