Blood & Immunology Physiology

^[1]

Meet the patient

A 62-year-old with pneumococcal sepsis is on day 1 in the ICU: febrile, acidotic, climbing lactate, on noradrenaline. Her oxygen–haemoglobin dissociation curve has shifted right — fever, acidosis and a rising 2,3-DPG all favour unloading, the tissue's own demand-driven rescue. Then she is transfused two units of stored blood for a haemoglobin of 68 g/L, and the curve snaps left: banked red cells are 2,3-DPG-depleted (their P50 falls from 26.6 toward 20 mmHg during storage) and hold their oxygen tightly, so tissue unloading transiently worsens even as the haemoglobin number climbs.^{[7][11][20][21]}

Day 12 of the same admission. She "looks better" — afebrile, off vasopressors, CRP falling — then spikes again: purulent secretions and new infiltrates (ventilator-associated pneumonia), blood cultures growing Candida, CMV PCR newly positive. The monocyte HLA-DR has fallen below 8000 molecules per cell. The immune system that fought the infection on day 1 is now functionally silent.^{[12][10][17][30]}

That is the single question this whole topic answers: why does the same patient die of too much inflammation on day 1 and too little immunity on day 12? Hold that question and every section below — the red cell, the complement, the clotting cascade, the SIRS–CARS swing — falls into place.^[10]

References32ShowHide

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