Mednotes

L05 Blood Leucocytes in disease

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aliasesL05 Blood Leucocytes in disease

14 min read

Overview

This lecture covers how leucocytes are counted and examined in practice, the two developmental lineages (myeloid and lymphoid), and the physiology, quantitative disorders and functional defects of each granulocyte/monocyte/lymphocyte type, before finishing with an introduction to how blood and marrow cancers are classified by lineage and maturity.

Examining blood

  • Full blood counts are now highly automated: analysers (e.g. Sysmex) process a sample and within about 30 seconds return a white cell count (WBC), red cell indices, platelets and a five-part differential (neutrophils, lymphocytes, monocytes, eosinophils, basophils), alongside scattergram plots that cluster cells by type using fluorescence and light scatter.
  • A blood film (stained smear on a slide) is examined manually in fewer than 15% of blood counts.
  • Bone marrow biopsy is performed about 300 times per year locally, sampling the posterior iliac crest. Two specimens are taken: a marrow aspirate (smear) and a marrow trephine (core biopsy). Normal marrow histology shows haematopoietic (blood-forming) cells interspersed with fat spaces and bone trabeculae.

Lineages: lymphoid vs myeloid

  • When thinking about blood, always separate the lymphoid series from the myeloid series. The concept of a single “total leukocyte count” is flawed and largely meaningless, since granulocytes and lymphocytes share almost nothing functionally except a purple-staining nucleus.
  • The pluripotent stem cell gives rise to two stem cell lines:
    • Lymphoid stem cell -> (via the thymus) T lymphocyte; also NK lymphocyte; also B lymphocyte -> plasma cell. Together these form the lymphoid series (lymphocytes).
    • Myeloid stem cell -> erythrocyte; megakaryocyte (platelet production/clotting); monocyte -> macrophage; granulocytes. Together these form the myeloid series.
  • Blood cancers are classified as arising from either the lymphoid or the myeloid lineage, using this same branching scheme.

Neutrophils: production and kinetics

  • Neutrophil maturation in the bone marrow (granulopoiesis) proceeds through a fixed sequence: myeloblast (rapid division, basophilic cytoplasm) -> promyelocyte (rapid division, primary granules) -> myelocyte (division stops, secondary granules develop) -> metamyelocyte (indented nucleus) -> band neutrophil -> mature neutrophil (segmented nucleus).
  • Kinetics: dividing cells (myeloblasts, promyelocytes) mature into non-dividing forms (myelocytes, band forms, neutrophils), which then circulate or marginate before entering tissues. Approximate timeline: about 10 days in the bone marrow, about 5 days in the blood, 3-5 days in tissues.
  • G(M)-CSF (granulocyte(-monocyte) colony-stimulating factor) acts from the dividing/maturation stage through to circulating neutrophils. G-CSF is commonly given clinically to stimulate neutrophil production after chemotherapy.

Neutrophil function

  • Core role: search for, ingest and destroy bacteria. This usually happens in tissues (e.g. pneumonia, appendicitis, a pimple); organisms are only very rarely seen inside neutrophils in the bloodstream itself. This is extremely rare and usually associated with death; one case featured diplococci (Neisseria meningitidis) in the blood of an 8-year-old child presenting with meningococcal sepsis, who was back at school a month later.
  • Neutrophil adhesion and migration follows a stepwise “migration cascade” at the endothelium: rolling (via selectins) -> slow rolling (selectins, integrins) -> arrest (chemokines, integrins, ICAM-1) -> crawling (integrins) -> transmigration into tissue. About half of circulating neutrophils are rolling along the endothelium at any time, a state called margination.
  • After transmigration, neutrophils move through tissue toward the target and act via phagocytosis, reactive oxygen species and degranulation; opsonins (IgG, complement) coat bacteria/targets to promote this. In sterile injury models (e.g. thermal liver injury), circulating neutrophils progressively accumulate at the injury site over several hours.
  • Neutrophil “destiny” in tissue: stimuli (inflammatory cytokines, opsonised bacteria, immune complexes, fungal/ECM components) drive the cell down one of two pathways:
    • NETosis: release of a chromatin lattice studded with antimicrobial peptides (a neutrophil extracellular trap, NET). Pro-inflammatory effects: releases self-antigens, stimulates interferon-alpha/beta, forms a chromatin lattice, presents antimicrobial peptides. NETs physically trap bacteria and yeast.
    • Apoptosis: the cell becomes a rounded apoptotic body. Anti-inflammatory effects: uptake by macrophages, stimulation of anti-inflammatory cytokines, removal of cellular debris.

Neutrophil disorders

  • Neutrophilia (increased neutrophils; reference interval 1.9-7.5 x10^9/L):
    • Infection (e.g. pneumonia, abscess, tonsillitis, urinary tract infection) - associated with a “shift to left” (increased immature neutrophil forms) and, in severe infection, toxic changes.
    • Steroid-induced neutrophilia: steroids cause demargination (marginated neutrophils re-enter the circulating pool).
    • Chronic mild neutrophilia can result from smoking, obesity and other causes.
  • Reactive neutrophil changes seen in severe infection: left shift (shift to immaturity), toxic granulation (increased number of prominent granules), and vacuolation (autophagocytosis/phagocytosis visible as cytoplasmic vacuoles).
  • Neutropenia (< 1.9 x10^9/L): causes include being part of a pancytopenia (anaemia + neutropenia + thrombocytopenia, e.g. marrow cancer/fibrosis, chemotherapy side effect), drug side effects (marrow suppression), viral infection (common, usually mild, e.g. glandular fever/EBV), genetic neutropenia (familial), the Duffy-null blood group (associated with lower neutrophil counts, e.g. African/Middle Eastern ancestry, and protective against malaria), and fulminant bacterial infection (uncommon but very important not to miss).
    • Mild neutropenia (1-1.9 x10^9/L): not clinically important; assessed by history (medication, chronicity, viral infection, Duffy-null ethnicity) and by checking the rest of the blood count and film.
    • Severe neutropenia (< 0.4 x10^9/L): watch closely for fever/elevated CRP, give IV antibiotics if febrile, consider protective isolation, occasionally treat with G-CSF.
  • Case (35-year-old, 9 weeks pregnant): serial blood counts showed a sharp fall in WBC (5.2 -> 1.5 x10^9/L) and neutrophils (2.7 -> 0.1 x10^9/L, flagged) alongside a platelet fall (257 -> 104, flagged) and mild haemoglobin/lymphocyte falls, with reticulocytes still within range (61, ref 10-100).
  • Follow-up 5 days later: CRP rose sharply (1 -> 23 mg/L) then fell (23 -> 10 -> 4), consistent with a viral infection (rather extreme in this case). Over three further counts the platelets (113 -> 169 -> 194) and neutrophils (0.1 -> 0.1 -> 1.1, still below the 1.9-7.5 reference range) both showed a recovery trend.
  • Functional defects of neutrophils:
    • Acquired: diabetes (impaired phagocytosis and killing), alcoholism (impaired phagocytosis and killing), renal failure (impaired phagocytosis).
    • Steroid medication: causes neutrophilia via demargination, but the released neutrophils are less able to adhere and fail to egress into tissues (functionally impaired despite a higher count).
    • Genetic defects in neutrophil function: can affect many of the functions described above, but are very rare.

Eosinophils

  • Eosinophilia: > 0.4 x10^9/L. Associated with parasitic infection and allergic reactions (including drug reactions); some autoimmune conditions and many rare blood cancers can also cause it. Eosinophils play multiple, broad roles in immunity and tissue homeostasis.
  • Case (26-year-old male, 10 days of diarrhoea, query worms): WBC markedly raised at 21.0 x10^9/L (ref 4.0-11.0), driven mainly by a very high eosinophil count of 8.5 x10^9/L (ref < 0.6), with neutrophils only mildly raised (7.9, ref 1.9-7.5) and other lines normal - a pattern suggestive of parasitic (worm) infection.

Monocytes and macrophages

  • Monocytes differentiate into macrophages (phagocytes) in almost all organs and tissues, with tissue-specific names: connective tissue (histiocytes), liver (Kupffer cells), lung (alveolar macrophages), brain (microglial cells), serous fluids (pleural and peritoneal macrophages), skin (Langerhans cells), bone (osteoclasts). Macrophages make up 10-15% of cells in every organ.
  • Marrow-derived macrophages give rise to brain microglial cells. In mice with Hoxb8 mutations, dysfunctional macrophages cause abnormal behaviour (overgrooming/bald patches); bone marrow transplant (replacing macrophage/microglial precursors) cures the overgrooming, showing that dysfunctional macrophages can cause abnormal behaviour and that transplanted normal microglial cells normalise it.
  • Macrophages actively phagocytose particulate material, illustrated by uptake of fungal spores and oil droplets.
  • Monocytosis: > 1.0 x10^9/L. Two main causes: (1) reactive chronic inflammatory states, e.g. untreated pneumonia, bacterial endocarditis, tuberculosis, brucellosis; (2) myelodysplastic syndrome (a bone marrow cancer of the elderly).
  • Case (85-year-old woman, very swollen knee, knee replacement 5 years ago): serial counts over three days showed a rising WBC (10.5 -> 14.1 -> 22.8 x10^9/L) with rising, markedly elevated neutrophils (9.4 -> 12.7 -> 18.7, ref 1.9-7.5), appearance of immature myelocytes/metamyelocytes by the third sample, low lymphocytes, and a rising monocyte count (0.7 -> 0.8 -> 1.8, ref 0.2-1.0).

Terminology for cell quantity

  • Too many cells: suffix “-osis” (Greek, “condition or state of,” especially abnormal), e.g. lymphocytosis, monocytosis, thrombocytosis, erythrocytosis; or “-philia” (Latin, “tendency”), e.g. neutrophilia, eosinophilia, basophilia.
  • Too few cells: suffix “-penia” (Latin “deficiency,” from Greek “wasting”), e.g. neutropenia, lymphopenia, thrombocytopenia.

Basophils

  • Basophils contribute to immunity against worms and help regulate chronic allergic inflammation.
  • The basophil count is seldom clinically useful.

Leukoerythroblastic state

  • A leukoerythroblastic blood film shows immature “white” and red cells in the blood (e.g. myelocytes, teardrop-shaped red cells, nucleated red blood cells) and is usually an important sign of severe bone marrow disease.
  • Immature cells are “pushed out” of a full or infiltrated marrow by: marrow replacement from leukaemia or lymphoma, marrow infiltration by metastatic cancer, or myelofibrosis (a marrow cancer). It can also occur in severe hypoxia combined with infection (e.g. pneumonia).

Lymphocytes

  • Three main lymphocyte types circulate in blood: T cells (about 60-80%, CD4+ or CD8+), B cells (about 15-30%, produce antibodies), and NK cells (about 5-10%). Lymphocytes show a wide range of morphology.
  • Lymphocytosis: its significance depends on age.
    • Children: viral infections, pertussis (whooping cough), acute lymphoblastic leukaemia (ALL).
    • Young adults: EBV (Epstein-Barr virus).
    • Any age: CMV (cytomegalovirus).
    • Older patients: less common causes include EBV, CMV, ALL; more common causes are chronic lymphocytic leukaemia (CLL) and some lymphomas (though most lymphomas do not show circulating cells).
  • Glandular fever (acute EBV infection): EBV infects B cells, but the large atypical lymphocytes seen in the blood are actually reactive T cells. “Infectious mononucleosis” is a misleading name since monocytes are not affected. Can occur at any age but is common in young adults. Features: fever (mild to severe), enlarged neck lymph nodes, sore throat, enlarged spleen, transient neutropenia, and mild hepatitis.

Blood and marrow cancers

  • General concepts: cancer reflects loss of the normal response to regulatory signals for growth, differentiation and death, and is a clonal proliferation (arising from a single mutated cell that expands into a large clonal population). Genetic mutations drive this by enhancing growth signals, disrupting growth regulators, and preventing cell death. A malignant tumour may replace normal marrow and infiltrate other tissues, destroying normal cells.
  • Blood cancers are classified as arising from either the lymphoid or myeloid lineage (as in the earlier lineage diagram).
  • Myeloid cancers (terminology given for familiarity, not exams):
    1. Acute myeloid leukaemia: rapid growth of precursor cells (myeloblasts) in the marrow; aggressive, fatal within weeks if untreated.
    2. Myeloproliferative neoplasms: excessive growth of mature cells in the marrow - mainly red cells gives polycythaemia vera (prognosis: decades), mainly platelets gives essential thrombocythaemia (decades), mainly granulocytes gives chronic myeloid leukaemia (decades, with modern treatment).
    3. Myelodysplastic syndrome: production of defective myeloid cells in the marrow; common in the elderly.
  • Lymphoid cancers, classified by the maturity and function of the cell of origin:
    • Precursor lymphoid neoplasms: acute lymphoblastic leukaemia (ALL) - rapid proliferation of large precursor cells (lymphoblasts).
    • Mature lymphoid neoplasms: lymphoma (usually a solid mass, proliferation of mature cells usually in lymph nodes); chronic lymphocytic leukaemia (CLL, slow proliferation of mature B cells in the blood); plasma cell myeloma (clonal proliferation of antibody-producing B cells/plasma cells).
    • Lymphoid cancers reflect the behaviour of their cell of origin along the B-cell maturation pathway (bone marrow pro-B/pre-B/immature B cell -> periphery: mantle zone mature/maturing B cell -> antigen stimulation -> follicular centre activated B cell -> memory B cell -> plasma cell): CLL corresponds to the mature/maturing B cell stage; ALL arises from precursor lymphocytes; follicular lymphoma and myeloma arise from mature lymphocytes.
  • Malignant disease in the marrow (e.g. leukaemia, lymphoma, metastatic cancer) can cause bone marrow failure leading to pancytopenia: anaemia (tiredness, shortness of breath), neutropenia (risk of infection), and thrombocytopenia (risk of bleeding).

One slide (an engagement/puzzle graphic showing letter-labelled cells on a blood film) is decorative and its exact intended phrase could not be determined from the transcript; it is omitted from the content above as it carries no factual teaching content.

Self-test

  1. Explain why the “total leukocyte count” is described as a flawed and largely meaningless concept, in terms of the lymphoid and myeloid lineages.
  2. Describe the sequence of cell stages in neutrophil maturation from myeloblast to mature neutrophil.
  3. Describe the steps of the neutrophil migration cascade from the circulation into tissue, and define margination.
  4. Distinguish NETosis from apoptosis as neutrophil “destinies” in tissue, including one effect of each.
  5. List three causes of neutrophilia and explain the mechanism behind steroid-induced neutrophilia.
  6. List four causes of neutropenia.
  7. What is the neutrophil count threshold that defines severe neutropenia, and what clinical actions does it warrant?
  8. Describe the reactive neutrophil changes seen in severe infection (three named changes).
  9. Explain how diabetes, alcoholism and steroid medication each impair neutrophil function, despite steroids raising the neutrophil count.
  10. What two categories of condition classically cause eosinophilia?
  11. List four tissue-specific names for macrophages and their locations.
  12. What are the two main causes of monocytosis?
  13. Distinguish the suffixes “-osis”/“-philia” from “-penia,” with one example each.
  14. What two functions are attributed to basophils?
  15. Define a leukoerythroblastic blood film and list two causes.
  16. Distinguish the causes of lymphocytosis in children versus in older patients.
  17. A young adult presents with fever, sore throat, enlarged neck nodes and splenomegaly with atypical lymphocytes on film. What is the likely diagnosis, which cells does the causative virus infect, and which cells do the atypical lymphocytes actually represent?
  18. Distinguish acute myeloid leukaemia from a myeloproliferative neoplasm in terms of growth pattern and prognosis.
  19. Distinguish acute lymphoblastic leukaemia from chronic lymphocytic leukaemia in terms of the maturity of the proliferating cell and the pace of disease.
  20. A patient with marrow failure from metastatic cancer develops pancytopenia. Explain the three resulting clinical problems and their mechanisms.

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