Neutrophils, Eosinophils, and Basophils
These cells have different functions and lifespans in the bloodstream, but all of them originate in the bone marrow.
Neutrophils form the core of the white blood cell population (40–75%). After formation, they circulate in the blood for only a couple of hours before migrating into tissues via chemotaxis. There, they engulf cellular debris and opsonized bacteria. Together with macrophages, neutrophils provide the primary nonspecific immune response. Their total lifespan is about eight days.
Eosinophils (1–5%) are granular leukocytes. After leaving the bone marrow and spending a few hours in the blood, they settle in the mucous membranes of the gastrointestinal, respiratory, and urogenital tracts. Their main task is participating in anti-parasitic, allergic, and inflammatory reactions. Eosinophils are capable of phagocytosis and also regulate inflammation by releasing a mast cell degranulation inhibitor and the enzyme histaminase (which inactivates histamine).
Basophils are the smallest group (0–1%). They live in the bloodstream for one to two days. Their granules store heparin and histamine. When activated, basophils migrate into tissues, degranulate, and trigger immediate-type hypersensitivity reactions by releasing vasoactive substances.
Monocytes and Tissue Macrophages
Monocytes (2–9%) are distinguished as the largest cells of the leukocyte series. After 2–4 days of circulation in the blood, they penetrate into tissues where they transform into macrophages — mobile cells found in all organs.
Macrophages live for months and perform critical tasks:
- Blood clearance: fixed cells of the liver, spleen, and bone marrow remove old erythrocytes and denatured proteins.
- Tissue phagocytosis: engulfment of matrix and cellular debris.
- Respiratory tract defense: alveolar macrophages phagocytose dust particles.
- Specific phagocytosis: destruction of opsonized bacteria.
Activated macrophages possess pronounced secretory function. They produce enzymes, interleukins, transport proteins, growth factors, and thromboxanes. The release of lysozyme provides an antibacterial effect, while the secretion of endogenous pyrogens participates in thermoregulation.
Lymphocytes and Adaptive Immunity
Lymphocytes (20–45%) live for many months and years, driving adaptive immunity. Originating in the red bone marrow, they undergo differentiation in primary lymphoid organs. Afterward, they constantly recirculate: moving from lymph nodes via lymph into the blood, entering tissues via diapedesis, and returning back.
Three main subpopulations are distinguished:
- T lymphocytes (80% or more). Mature in the thymus, participating in cell-mediated and humoral immunity. Cytotoxic T cells (T-killers) mediate cellular responses, while T-helper cells stimulate the generation of cytotoxic T lymphocytes, maintain suppressor T-cell activity, and assist in the proliferation and differentiation of B lymphocytes.
- B lymphocytes (less than 10%). Differentiate in the bone marrow. Upon encountering an antigen, they migrate to secondary lymphoid organs and transform into plasma cells. Each clone produces antibodies specific to a single antigen, forming humoral immunity and immune memory.
- NK cells (Natural Killer cells). Make up 5–10% of circulating lymphocytes. They lack surface determinants characteristic of T and B cells. Their role is the destruction of foreign, virus-infected, and tumor (transformed) cells.