🧬 Origin · Hematopoiesis
Hematopoiesis is the lifelong process of blood cell production, differentiation, and maturation from multipotent hematopoietic stem cells (HSCs). It ensures a constant supply of erythrocytes, megakaryocytes, and all leukocyte types.
- Embryonic – yolk sac (first weeks).
- Fetal – liver and spleen become dominant during organogenesis.
- Adult – bone marrow (axial skeleton, proximal long bones) is the primary site throughout life.
Asymmetric division & potency
HSCs undergo asymmetric division: one daughter cell remains a stem cell (self‑renewal), the other becomes a common lymphoid progenitor or common myeloid progenitor. Cytokines and growth factors (SCF, TPO, IL‑3, GM‑CSF) steer differentiation.
🔵 Lymphoid & Myeloid lineages
🔵 Lymphoid
- Common lymphoid progenitor →
- B lymphocytes (develop in bone marrow)
- T lymphocytes (migrate to thymus for maturation)
- Natural killer (NK) cells
Adaptive B and T cells mediate antigen‑specific responses. NK cells are innate large granular lymphocytes that kill virus‑infected and tumor cells without prior sensitization.
🟢 Myeloid
- Common myeloid progenitor →
- Erythrocytes, megakaryocytes (platelets)
- Granulocytes: neutrophils, eosinophils, basophils
- Monocytes / macrophages, dendritic cells, mast cells
Innate Myeloid cells perform rapid, stereotypic responses: phagocytosis, killing of pathogens, antigen presentation, and allergic/inflammatory reactions.
🩸 White blood cells · morphology & function
Below is a functional summary of the major leukocyte types. Morphologic features (nucleus shape, granules) are key for differential identification.
| Cell type | Lineage | Key morphology | Primary function |
|---|---|---|---|
| Neutrophil (PMN) | Myeloid | Segmented (3–5 lobes), pale pink granules | Phagocytosis of extracellular bacteria & fungi; first responder |
| Eosinophil | Myeloid | Bilobed nucleus, large pink/orange granules | Helminth defense; type I hypersensitivity (allergy) |
| Basophil | Myeloid | Bilobed (often obscured), large deep‑blue granules | Allergic reactions; release histamine, heparin |
| Monocyte | Myeloid | Kidney‑shaped nucleus, agranular | Circulating precursor to macrophage / dendritic cell |
| Macrophage | Myeloid | Ruffled membrane, vacuoles, irregular shape | Phagocytosis, APC, tissue resident (Kupffer, alveolar, etc.) |
| Dendritic cell | Myeloid | Stellate cytoplasmic projections (“dendrites”) | Professional APC; T‑cell activation; bridging innate & adaptive |
| Mast cell | Myeloid | Small nucleus, abundant deep‑blue granules | Type I hypersensitivity; parasite defense; tissue resident |
| B lymphocyte | Lymphoid | Large nucleus, thin cytoplasm | Antibody production (plasma cells), antigen presentation |
| T lymphocyte | Lymphoid | Large nucleus, thin rim of cytoplasm | Cell‑mediated immunity: CD4+ helper, CD8+ CTL |
| NK cell | Lymphoid | Large granular lymphocyte | Innate killing of virus‑infected cells & tumor cells |
🧫 B, T, and NK lymphocytes
- B lymphocytes – mature in bone marrow; express surface immunoglobulin (BCR). Upon activation → plasma cells (antibody factories).
- T lymphocytes – progenitor migrates to thymus for positive/negative selection; TCR recognition of peptide‑MHC. CD4+ (helper) and CD8+ (cytotoxic).
- NK cells – large granular lymphocytes; recognize downregulated MHC class I or stress ligands; kill via perforin/granzyme; no rearrangement of antigen receptors.
📊 WBC differential · reference ranges
Laboratory evaluation includes absolute and relative counts. Shifts in proportions suggest infection, allergy, or hematologic disorders.
| Cell type | Adult reference range (%) |
|---|---|
| Neutrophils (PMNs) | 50–70 |
| Band cells (immature neutrophils) | 0–5 |
| Lymphocytes | 20–40 |
| Monocytes | 5–10 |
| Eosinophils | 0–5 |
| Basophils | <1 |
💎 Clinical pearls & high‑yield facts
- Plasma cells – terminally differentiated B lymphocytes; eccentrically placed nucleus, prominent Golgi, abundant antibodies.
- Dendritic cells – most potent APC; express MHC class II and co‑stimulatory molecules.
- Mast cells vs Basophils – both contain histamine; mast cells are tissue‑resident, basophils circulate.
- NK cells – lack CD3, do not rearrange antigen receptors; part of innate immunity.
- Macrophages – arise from monocytes; tissue‑specific names (microglia, Kupffer, alveolar).