Unit 08 — Blood · Question Bank

Blood is best understood as a fluid connective tissue: a liquid extracellular matrix (plasma, ~55%) suspending a population of formed elements (~45%) — erythrocytes, leukocytes and platelets. A peripheral smear stained with a Romanowsky cocktail (Wright's, Giemsa or Leishman) reveals every type by colour and shape, and the whole classification grows out of one decision tree: does the cell have a nucleus, and does its cytoplasm carry specific granules?

Erythrocytes — the gas carriers

The erythrocyte is the most peculiar cell in the body because it has discarded almost everything that defines a normal cell. During its final maturation in the bone marrow it ejects its nucleus and digests its organelles, leaving only a plasma membrane wrapped around a concentrated solution of haemoglobin. The shape that results is a biconcave disc ~7.5 µm across, thin in the middle (producing the central pallor on H&E), with a remarkable surface-to-volume ratio and the deformability needed to squeeze through capillaries narrower than itself. Membrane integrity is maintained by a hexagonal cytoskeletal mesh of spectrin tied to the membrane by ankyrin and band 4.1 — the substrate whose defect underlies hereditary spherocytosis. The cell circulates for ~120 days, then is removed by splenic macrophages and its iron is recycled.

Granulocytes — innate defence with specific granules

The granulocytes form the polymorphonuclear arm of innate immunity. The neutrophil (10–12 µm, 60–70%) is the most abundant of all leukocytes, defined by a 2- to 5-lobed nucleus joined by thin chromatin threads, with two granule populations — fine lilac specific granules (lysozyme, lactoferrin) and azurophilic lysosomes (myeloperoxidase) — that together execute the full kill cycle against bacteria. The eosinophil (12–15 µm, 1–4%) carries a bilobed nucleus and coarse crimson refractile granules whose crystalline core contains major basic protein, the parasiticide that also modulates allergic reactions. The basophil (10–14 µm, <1% — the rarest leukocyte) has an irregular nucleus obscured by dark blue-purple metachromatic granules of histamine and heparin, the same payload as the tissue mast cell, driving immediate hypersensitivity.

Agranulocytes — adaptive intelligence and tissue cleanup

The lymphocyte (6–8 µm, 20–30%) looks deceptively simple — a dense round nucleus nearly filling the cell with only a thin sky-blue cytoplasmic rim and no specific granules — yet it carries the entire adaptive immune memory: B cells (antibody production), T cells (helper and cytotoxic responses) and NK cells (innate killing). The monocyte (14–20 µm, 3–8% — the largest leukocyte) has a kidney-shaped nucleus and pale grey-blue cytoplasm with fine azurophilic dust; it is a cell in transit and, on leaving the vessel, becomes a tissue macrophage of the mononuclear phagocyte system (Kupffer cell in liver, microglia in brain, alveolar macrophage in lung, osteoclast in bone).

Platelets — the haemostatic plug

Platelets are not whole cells but 2–4 µm anucleate cytoplasmic fragments shed from giant polyploid megakaryocytes in the bone marrow. On smear each shows a pale peripheral hyalomere (microtubule + actin cytoskeleton) and a darker central granulomere with α-granules (vWF, fibrinogen, PDGF) and dense δ-granules (ADP, ATP, serotonin, Ca²⁺). On vessel injury they adhere to exposed collagen through GPIb/vWF, activate, change shape, secrete ADP to recruit more platelets and form the primary haemostatic plug on which the coagulation cascade lays down fibrin.

Clinical anchor

Each formed element has its diseases: anaemia (erythrocyte deficit), neutrophilia/neutropenia (bacterial infection or marrow failure), eosinophilia (allergy, asthma, helminths), basophilia (CML), lymphocytosis (viral infection, CLL), monocytosis (chronic infection, endocarditis) and thrombocytopenia (ITP, DIC). Reading a CBC is reading the formed elements one by one.

Granulocytes are the polymorphonuclear leukocytes of innate immunity, defined by the presence of conspicuous specific (secondary) granules in their cytoplasm. Three cell types share this morphology — neutrophil, eosinophil and basophil — and despite a common ancestor in the bone marrow (the myeloblast), each has specialised in a different defensive role: bacterial killing, anti-parasitic and anti-allergic action, and immediate hypersensitivity.

Neutrophil — the bacterial first responder

The neutrophil (10–12 µm, 60–70% of circulating leukocytes) is the most abundant nucleated cell in your blood. Its multilobed nucleus — 2 to 5 dense lobes joined by thin chromatin bridges — is its signature on a smear; the lobulation is functional, allowing the cell to fold and thread through capillaries that a spherical nucleus would block. Its cytoplasm holds two granule populations. The specific (secondary) granules are small, neutral-staining (the reason the cell is called neutrophil) and packed with lysozyme, lactoferrin and collagenase — antimicrobial weapons that act outside the cell on bacterial walls. The azurophilic (primary) granules are larger lysosomes carrying myeloperoxidase and acid hydrolases for the kill inside the phagosome. Together they execute the full sequence: chemotaxis, diapedesis, phagocytosis, oxidative burst, degranulation, apoptosis. Dead neutrophils with the bacteria they trapped are what you see clinically as pus.

Eosinophil — parasite killer and allergy modulator

The eosinophil (12–15 µm, 1–4%) is unmistakable: a bilobed nucleus that looks like a pair of spectacles, and a cytoplasm crammed with coarse crimson refractile granules. Under EM each granule shows a striking crystalline core of major basic protein set in a less dense matrix — pure pathognomonic morphology. MBP is a parasiticide: it punches holes in the cuticle of helminthic worms. Eosinophils also dampen allergic reactions by neutralising histamine and degrading leukotrienes — which is why they rise both in worm infections and in atopic disease (the “NAACP” mnemonic).

Basophil — immediate hypersensitivity

The basophil (10–14 µm, <1%) is the rarest leukocyte in blood. Its irregular S-shaped nucleus is usually obscured by large, dark blue-purple metachromatic granules — they shift the colour of certain dyes — that contain histamine and heparin, the same payload as the tissue mast cell. Both cell types carry the high-affinity IgE receptor FcεRI; when IgE is crosslinked by allergen, they degranulate to drive immediate (type I) hypersensitivity — the urticaria, bronchospasm and anaphylaxis of clinical allergy.

Origin and granule maturation

All three derive from the myeloblast in red bone marrow through a series of stages — promyelocyte, myelocyte, metamyelocyte, band, mature granulocyte — with azurophilic granules appearing at the promyelocyte stage and specific granules appearing at the myelocyte stage. The myelocyte is the last dividing form; band and mature stages can no longer replicate.

Clinical anchor

Neutrophilia with a left shift (band forms) signals bacterial sepsis; neutropenia after chemotherapy raises the febrile-neutropenia emergency. Eosinophilia points to allergy, drug reaction, or parasitic disease; basophilia is a classic clue to chronic myeloid leukaemia (BCR-ABL).

The agranulocytes of blood are the lymphocytes and the monocytes — the two leukocyte populations whose cytoplasm lacks conspicuous specific (secondary) granules. Both still carry sparse azurophilic lysosomes, but the absence of the brightly staining specific granules of the neutrophil, eosinophil and basophil makes them visually quieter on a Romanowsky-stained smear. That visual quietness is misleading: between them, lymphocytes and monocytes carry the body's adaptive immune memory and seed virtually every macrophage population in the tissues.

Lymphocyte — small cell, vast repertoire

The small lymphocyte (6–8 µm — the smallest leukocyte, barely larger than an erythrocyte) is dominated by a dense, round, deeply basophilic nucleus that nearly fills the cell, leaving only a thin sky-blue rim of cytoplasm and no specific granules. The cell looks almost like a nucleus with a coat painted around it. This is functional: the lymphocyte is essentially an archive, and the cytoplasm is sparse because the work is mental, not muscular. Activated and large lymphocytes (10–12 µm) appear during immune responses with more cytoplasm and looser chromatin.

Functionally the lymphocytes split into three lineages that are indistinguishable on light microscopy — flow cytometry is needed to separate them. B lymphocytes mature in the bone marrow and, on antigen encounter, differentiate into antibody-producing plasma cells (humoral immunity). T lymphocytes mature in the thymus and divide into CD4+ helpers (orchestrate the response) and CD8+ cytotoxics (lyse virally infected and tumour cells — cell-mediated immunity). NK cells kill infected and tumour cells without prior sensitisation as part of innate immunity. The lymphocyte is therefore the single cell on which the entire concept of adaptive immunity rests.

Monocyte — the largest leukocyte, a cell in transit

The monocyte (14–20 µm) is the largest cell on a normal blood smear. Its nucleus is unmistakable: a deep kidney or horseshoe shape, often appearing as if a bite has been taken from one side. The cytoplasm is pale grey-blue with fine azurophilic dust — those granules are lysosomes, primed for action. The monocyte in blood is a cell in transit. It crosses the endothelium by diapedesis, enters tissue, swells, accumulates lysosomes, and differentiates into a macrophage. Once in tissue, it acquires a site-specific name — Kupffer cell in liver, microglia in brain, alveolar macrophage in lung, osteoclast in bone (after fusion), Langerhans cell in skin, dust cell in lung. All of these are members of the mononuclear phagocyte system.

Function and comparison

Both agranulocytes therefore complement the granulocyte response. Where neutrophils provide rapid, non-specific, short-lived bacterial killing, lymphocytes provide slower, antigen-specific, long-memory immunity, and monocyte-derived macrophages provide sustained tissue-resident phagocytosis, antigen presentation and cytokine signalling that direct the entire immune effort. The split between innate and adaptive arms is essentially the split between granulocytes/monocytes and lymphocytes.

Clinical anchor

Lymphocytosis — viral infections (EBV, CMV), pertussis, chronic lymphocytic leukaemia (smudge cells on smear). Lymphopenia — HIV (CD4 destruction), corticosteroid therapy, sepsis. Acute lymphoblastic leukaemia (ALL) is the commonest paediatric cancer. Monocytosis — chronic infection (TB, infective endocarditis), connective-tissue disease and certain leukaemias.

The erythrocyte is the strangest cell in the body, and the strangeness is the whole point. During its final maturation in the bone marrow it deliberately ejects its nucleus and digests every organelle. What remains is a plasma membrane wrapped around a concentrated solution of haemoglobin — a pure gas-transport sack engineered for one job. Every feature of its structure, from shape to cytoskeleton to lifespan, follows from that specification.

Shape and dimensions

The mature erythrocyte is a biconcave disc ~7.5 µm across, thin in the middle and slightly thicker at the rim. On an H&E or Romanowsky smear it appears as a uniform pink ring with a pale central pallor, because the thin middle holds less haemoglobin between the two depressed membranes. The biconcavity is elegant geometry: it maximises surface-to-volume ratio (good for rapid gas exchange) and lets the cell fold like an umbrella to enter capillaries narrower than its own diameter. The 7.5 µm diameter is so reliable that pathologists use it as a built-in ruler on a smear — any nucleus the same size as an RBC is “small.”

Anucleate, organelle-free, haemoglobin-filled cytoplasm

The cell carries no nucleus and no organelles — no mitochondria, no Golgi, no rough ER, no ribosomes. Cytoplasm is almost entirely haemoglobin (~33% of cell mass), plus carbonic anhydrase to handle CO₂ transport. Anucleation maximises the intracellular volume available to haemoglobin; elimination of mitochondria prevents the cell from consuming the oxygen it is meant to deliver, so the erythrocyte relies entirely on anaerobic glycolysis for ATP. The shape also has a downside — without a nucleus the cell cannot repair damage or replenish enzymes, which sets a hard ceiling on its lifespan.

Membrane cytoskeleton

The biconcave shape and the cell's remarkable deformability are held together by a membrane cytoskeleton you must know by name. A hexagonal mesh of spectrin dimers lies just under the inner leaflet of the plasma membrane and is tied to integral membrane proteins through ankyrin (to band 3) and band 4.1 (to glycophorin). Knock out any of those anchor links and the cell rounds up into a sphere, loses its flexibility, jams in splenic cords and is destroyed prematurely — the entire pathophysiology of hereditary spherocytosis is captured in that one sentence.

Function — reciprocal gas transport

Each erythrocyte carries ~280 million haemoglobin tetramers, each binding four oxygen molecules. In the alveolar capillary, oxygen loads cooperatively onto Hb; in the tissue capillary, the drop in Pο₂ combined with rising CO₂ and H⁺ (the Bohr effect) releases oxygen. CO₂ is carried back partly as bicarbonate (formed inside the RBC by carbonic anhydrase, exchanged out via band 3) and partly bound to Hb as carbamino-Hb. The biconcave shape minimises the diffusion distance from membrane to Hb, making the exchange near-instantaneous.

Lifespan and clearance

The erythrocyte circulates for ~120 days. As it ages, its membrane stiffens, surface markers (eat-me signals like exposed phosphatidylserine) accumulate, and the spleen's red-pulp macrophages recognise and phagocytose it through the slow cords. Iron is released to transferrin and recycled (90% of body iron use); the porphyrin ring is broken down to biliverdin and bilirubin and excreted in bile.

Clinical anchor

Erythrocyte diseases follow directly from structure. Sickle-cell disease — a single Glu→Val substitution in β-globin causes HbS to polymerise when deoxygenated; cells distort into rigid crescents, jam capillaries, lyse early. Hereditary spherocytosis — spectrin/ankyrin/band 4.1 defect → sphering → splenic destruction. Thalassaemia — under-production of an α or β globin chain. G6PD deficiency — oxidative stress (fava beans, primaquine, sulphonamides) precipitates Hb as Heinz bodies and the spleen bites them out. Anaemia of chronic kidney disease — loss of erythropoietin from peritubular fibroblasts, treated with recombinant EPO.

Haemopoiesis is the continuous production of all the formed elements of blood from a single rare ancestor — the pluripotent haemopoietic stem cell. No cell circulating in your veins today is more than a few months old; the entire blood compartment is constantly renewed, and the factory keeps moving locations as you grow. To answer this question well you must walk through three things: the changing sites of haemopoiesis through life, the architecture of red bone marrow, and the three downstream lineages of erythropoiesis, granulopoiesis and thrombopoiesis.

Sites of haemopoiesis through life

In the first trimester blood forms in the embryonic yolk sac — primitive nucleated erythroblasts that look more amphibian than human. In the second trimester the work moves to the liver and spleen; fetal liver biopsies show clear marrow-style islands of developing cells. From the seventh fetal month onwards red bone marrow takes over, and at birth essentially every bone in the body contains active red marrow. As the child grows, red marrow retreats from the long bones (which turn yellow with fat) and consolidates in the axial skeleton — vertebrae, sternum, ribs, pelvis, skull, proximal femur and proximal humerus — which is why a bone-marrow biopsy is taken from the posterior iliac crest in adults. Under severe haematological stress (thalassaemia, myelofibrosis, chronic haemolysis) the liver and spleen can reactivate — extramedullary haemopoiesis — and enlarge.

Red bone marrow architecture and the stem cell

Red marrow is composed of two interlocking compartments: haemopoietic cords (islands of developing cells on a reticular-fibre mesh with fat cells) and vascular sinusoids (wide, thin-walled, fenestrated capillaries that drain into a central vein). Mature cells leave the cords by squeezing through endothelial fenestrations into the sinusoid — the same trans-endothelial trick a leukocyte uses in the opposite direction at a site of infection. All lineages descend from the pluripotent haemopoietic stem cell (HSC, CD34+), which commits first to either a common myeloid progenitor (erythroid + granulocyte/monocyte + megakaryocyte) or a common lymphoid progenitor (T, B, NK). Lineage-specific cytokines — erythropoietin (kidney) for RBC, thrombopoietin (liver/kidney) for platelets, G-CSF / GM-CSF for granulocytes, IL-3 / IL-7 / SCF for multiple lineages — steer each commitment step.

Erythropoiesis

The erythroid line goes through five named stages, each smaller, more condensed, less basophilic and more haemoglobin-rich than the last: proerythroblast → basophilic erythroblast → polychromatophilic erythroblast → orthochromatic erythroblast (normoblast) → reticulocyte → erythrocyte. The polychromatophilic stage is the last that can divide; the orthochromatic stage is the last with a nucleus, which it then extrudes. The reticulocyte enters the blood with residual ribosomes (the “reticulum” on supravital stain) and matures fully in ~1 day. The whole sequence is paced by erythropoietin from renal peritubular fibroblasts in response to tissue hypoxia — the feedback loop that fails in chronic kidney disease.

Granulopoiesis and thrombopoiesis

The granulocyte line passes through myeloblast → promyelocyte → myelocyte → metamyelocyte → band → mature granulocyte. Azurophilic granules appear at the promyelocyte stage; specific granules appear at the myelocyte stage, which is the last dividing form. Under bacterial stress the marrow releases band forms early — the “left shift” on a CBC. Thrombopoiesis is unique: a single megakaryocyte undergoes endomitosis (DNA replication without cytokinesis) to become a giant polyploid cell with a multilobed nucleus, then extends pro-platelet processes through sinusoidal endothelium and sheds thousands of platelets directly into the blood. Thrombopoietin (TPO) is the master regulator.

Clinical anchor

Lineage failure or excess defines whole categories of haematological disease: anaemia (erythroid failure), leukaemia (myeloid or lymphoid clonal proliferation — AML, CML with the Philadelphia chromosome, ALL, CLL), polycythaemia vera (JAK2-driven RBC excess), essential thrombocythaemia (platelet excess), aplastic anaemia (pancytopenia from stem-cell failure).