Blood Histology

Erythrocytes are biconcave, anucleate discs (7.5 µm diameter) optimised for gas transport and flexibility through capillaries. Their lifespan is ~120 days. Platelets are small, anucleate cell fragments derived from megakaryocytes in the bone marrow, crucial for haemostasis. Leucocytes are the immune component. Neutrophils (the most common, 40-75%) have multi-lobed nuclei and are the first responders to bacterial infection. Eosinophils (1-6%) have bilobed nuclei and pink-staining granules, associated with parasites and allergy. Basophils (<1%) have large purple granules containing histamine. Lymphocytes (20-45%) vary in size and mediate adaptive immunity (B and T cells). Monocytes (2-10%) are the largest leucocytes, featuring kidney-shaped nuclei; they migrate into tissues to become macrophages. Plasma contains water, proteins (albumin, globulins, clotting factors), and electrolytes. Bone marrow histology reveals haematopoiesis, showing precursors like erythroblasts, myeloblasts, and giant megakaryocytes against a background of adipocytes and reticular fibres.

Haematopoiesis begins with a pluripotent stem cell in the bone marrow. Erythropoiesis is regulated by erythropoietin (EPO) from the kidney. The biconcave shape of RBCs is maintained by a cytoskeleton involving proteins like spectrin and actin. Neutrophils contain primary (azurophilic) and secondary (specific) granules that contain enzymes like myeloperoxidase and lysozyme. Blood films are prepared using Romanowsky-type stains (e.g., Leishman or Wright-Giemsa), which differentiate cells based on their affinity for acidic (eosin) or basic (methylene blue) dyes. Lymphocytes cannot be morphologically distinguished as B or T cells without immunohistochemistry for CD markers (e.g., CD3 for T cells, CD19/20 for B cells).

Peripheral blood films are diagnostic in many conditions: Sickle cell disease showing crescent-shaped RBCs; Iron deficiency showing microcytic hypochromic cells; B12/Folate deficiency showing hypersegmented neutrophils and macrocytes. A 'left shift' (increase in immature neutrophils) suggests acute infection. Leukemia is characterised by the presence of 'blast' cells in the blood or marrow. Thrombocytopenia (low platelets) presents with petechiae and mucosal bleeding. Malaria can be diagnosed by seeing parasites (e.g., Ring forms) inside erythrocytes.

Full Blood Count (FBC) provides quantitative data. Peripheral Blood Film (PBF) provides qualitative morphology. Bone marrow aspirate and trephine biopsy assess the 'factory' of blood production. Common findings include Howell-Jolly bodies (post-splenectomy) and Schistocytes (haemolysis).

Not directly applicable to this basic-science topic; management depends on the specific haematological diagnosis (e.g., iron replacement, chemotherapy, or steroids).