While monitoring a patient receiving a blood transfusion, the nurse notes an abnormal drop in blood pressure to 90/50 mm Hg and an increase in heart rate to 118 beats per minute. The patient reports chills and lower back pain. Which type of hypersensitivity reaction is the patient experiencing?

Choice A rationale Type I hypersensitivity, or immediate hypersensitivity, is IgE-mediated and typically presents rapidly with symptoms like urticaria (hives), angioedema, bronchospasm, and hypotension due to massive mast cell degranulation and histamine release. While it can cause hypotension, the classic triad of chills, lower back pain, and profound hypotension/tachycardia is not primarily characteristic of this type during a blood transfusion. Choice B rationale Type II hypersensitivity, a cytotoxic reaction, is the classic mechanism for an acute hemolytic transfusion reaction (AHTR). The patient's pre-existing antibodies (IgG or IgM) bind to antigens on the transfused red blood cells (RBCs), leading to complement activation. This massive intravascular hemolysis releases inflammatory mediators, causing the characteristic triad of chills, back pain (due to renal damage/ischemia), hypotension, and tachycardia. Choice C rationale Type III hypersensitivity is an immune-complex reaction where soluble antigens and antibodies (IgG or IgM) form complexes that deposit in tissues, often manifesting as vasculitis, arthritis, or nephritis. While it can occur in delayed transfusion reactions, it is not the primary mechanism for the immediate, severe AHTR symptoms described, which are dominated by cell lysis. Choice D rationale Type IV hypersensitivity is a delayed, cell-mediated reaction involving T-lymphocytes and macrophages. It typically takes 24 to 72 hours to manifest and is responsible for contact dermatitis or the tuberculin skin test. The acute, life-threatening symptoms observed within minutes of a transfusion reaction exclude this slower, non-antibody-mediated response.