Clinical meaning
Aplastic anemia is a rare but serious bone marrow failure syndrome characterised by pancytopenia (simultaneous reduction of all three blood cell lines: red blood cells, white blood cells, and platelets) resulting from destruction or suppression of haematopoietic stem cells in the bone marrow. The bone marrow, which is normally packed with blood-forming cells, becomes hypocellular and replaced by fat. This results in profoundly inadequate production of all mature blood cells. The pathophysiology involves immune-mediated destruction of haematopoietic stem and progenitor cells. In most acquired cases (approximately 70-80%), autoreactive cytotoxic T lymphocytes (CD8+ T cells) attack and destroy bone marrow stem cells through direct cytotoxicity and production of inhibitory cytokines (interferon-gamma, TNF-alpha). These cytokines suppress stem cell proliferation and induce apoptosis. The trigger for this autoimmune attack is often unknown (idiopathic aplastic anemia), but identified triggers include certain medications (chloramphenicol, carbamazepine, phenytoin, sulfonamides, NSAIDs), chemical exposures (benzene, pesticides, organic solvents), viral infections (hepatitis, Epstein-Barr virus, HIV, parvovirus B19), and radiation exposure. The consequences of pancytopenia are predictable from each cell line deficiency. Anemia (low red blood cells) causes fatigue, pallor, dyspnoea,...