Neutrophils, eosinophils, and basophils
These cells have different functions and lifespans in the bloodstream, but they are all produced in the bone marrow.
Neutrophils make up the majority of the white blood cell population (40–75%). After formation, they circulate in the bloodstream for just a few hours and then migrate into tissues via chemotaxis. There, they engulf tissue debris and opsonized bacteria. Together with macrophages, neutrophils provide the primary non-specific immune response. Their total life cycle is about eight days.
Eosinophils (1–5%) belong to granulocytic leukocytes. After leaving the bone marrow and spending a few hours in the blood, they settle in the mucous membranes of the gut, respiratory, and urogenital tracts. Their main task is participating in antiparasitic, allergic, and inflammatory responses. 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 least numerous 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 an immediate-type hypersensitivity reaction through the release of vasoactive substances.
Monocytes and tissue macrophages
Monocytes (2–9%) stand out as the largest cells in the leukocyte series. After 2–4 days of circulation in the blood, they penetrate tissues, where they transform into macrophages—mobile cells found in all organs.
Macrophages live for months and perform crucial tasks:
- Blood purification: fixed cells of the liver, spleen, and bone marrow remove aged erythrocytes and denatured proteins.
- Tissue phagocytosis: engulfment of matrix and cellular debris.
- Respiratory tract defense: alveolar macrophages engulf dust particles.
- Specific phagocytosis: destruction of opsonized bacteria.
Activated macrophages exhibit pronounced secretory activity. 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 specific immunity
Lymphocytes (20–45%) live for many months and years, mediating specific immunity. Originating in the red bone marrow, they undergo differentiation in primary lymphoid organs. Afterwards, they continuously recirculate: moving from lymph nodes via lymph into the blood, entering tissues through diapedesis, and returning back.
Three main subpopulations are distinguished:
- T lymphocytes (80% or more). They mature in the thymus and participate in cellular and humoral immunity. Cytotoxic T cells (T killers) are responsible for the cellular response, while T helper cells stimulate the formation of cytotoxic T lymphocytes, maintain the activity of suppressor T cells, and assist in the proliferation and differentiation of B lymphocytes.
- B lymphocytes (less than 10%). They differentiate in the bone marrow. Upon encountering an antigen, they migrate to secondary lymphoid organs and transform into plasma cells. Each clone produces antibodies strictly against a single antigen, forming humoral immunity and immunological memory.
- NK cells (Natural Killer cells). They 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.
Briefly: answers to topical questions
What is the normal leukocyte differential count in an adult?
The normal leukocyte differential count in an adult represents the percentage ratio of various leukocyte types in the blood.
- Neutrophils — 40–75% of all leukocytes (normally represented by band and segmented forms).
- Lymphocytes — 20–45% of the total count.
- Monocytes — 2–9% of circulating leukocytes.
- Eosinophils — 1–5% of circulating cells.
- Basophils — 0–1% of the total leukocyte count.
What maturation stages do granulocytes undergo in the red bone marrow?
Granulocytopoiesis features sequential stages of morphological differentiation:
- Myeloblast.
- Promyelocyte — the next stage after the myeloblast; the cytoplasm contains primary (azurophilic) granules, while specific granules are not yet present.
- Myelocyte — secondary specific granules appear; myelocytes are the last cells of the granulocytopoietic series capable of cell division.
- Metamyelocyte.
- Band granulocyte.
- Segmented granulocyte.
During maturation, specific granularity appears in the cytoplasm, the nucleus condenses, and its shape changes from round to segmented.
To memorize the normal percentage ratio of leukocytes in descending order (Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils), use the mnemonic.
Class and exam questions
Which cells provide the primary non-specific immune response?
This task is jointly performed by neutrophils and macrophages, which phagocytose bacteria and cellular debris.
Where do lymphocytes undergo differentiation?
Precursors from the red bone marrow undergo differentiation in primary lymphoid organs. T lymphocytes mature in the thymus, while B lymphocytes mature in the bone marrow.
What is the difference between monocytes and macrophages?
Monocytes are cells circulating in the blood (for about 2–4 days). When they migrate into tissues, they transform into macrophages, which can function there for months.
What else to study on this topic
- Mechanisms of leukocyte chemotaxis and diapedesis in tissues
- The process of phagocytosis and the role of bacterial opsonization
- Secretory activity of macrophages (types of secreted interleukins and growth factors)
- Pathophysiology of immediate-type hypersensitivity reactions (basophil degranulation)
- Formation of B lymphocyte clones with immunological memory
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