What is the most likely source of stimulation of the anti-Rh (Rhesus) antibody in this patient?

Most Likely Source of Anti-Rh Antibody Stimulation

Transfusion with RH+ platelets is the most likely source of stimulation of the anti-Rh antibody in this patient (option C).

Mechanisms of Rh Alloimmunization

Platelet transfusions can cause Rh alloimmunization despite platelets not expressing Rh antigens on their surface. This occurs because:

• Platelet concentrates contain small amounts of contaminating red blood cells (RBCs)
• The minimum RBC volume needed to elicit a primary anti-D immune response is only 0.03 mL 1
• Whole-blood-derived platelet products contain approximately 0.036 mL of RBCs
• Apheresis platelet products contain approximately 0.00043 mL of RBCs 1
• RBC-derived microparticles in platelet concentrates may also stimulate antibody formation 2

Evidence Supporting Platelet Transfusion as the Source

The American Society of Clinical Oncology (ASCO) guidelines document that:

• Anti-D antibodies can be detected in 7.8% to 19% of RhD-negative patients with cancer exposed to RhD antigens via transfusions 1
• In a large retrospective study of 485 RhD-negative patients who received at least one platelet concentrate from an RhD-positive donor, 1.44% developed primary anti-D alloimmunization 1
• Even apheresis platelets, which have much lower RBC contamination, can provoke non-D Rh antibody formation 2

Why Other Options Are Less Likely

A. RH- pregnancy with failed RH prophylaxis

• This would require the patient to be Rh-negative and pregnant with an Rh-positive fetus
• Failed Rh prophylaxis is relatively uncommon when properly administered
• The question suggests an existing antibody rather than a failure to prevent one

B. RH+ pregnancy without RH prophylaxis

• While this is a common cause of Rh sensitization in the general population
• The context of the question (discussing various transfusion options) suggests a transfusion-related scenario

D. Transfusion with RH+ red cell components

• While this would definitely cause Rh sensitization
• The question appears to be distinguishing between different transfusion components
• Transfusion of Rh-incompatible red cells would be a more obvious error than platelet transfusion

Prevention Strategies

For RhD-negative patients receiving platelet transfusions:

• Prevention of RhD alloimmunization can be achieved through:

  1. Exclusive use of platelets from RhD-negative donors
  2. Administration of anti-D immunoprophylaxis (RhIG) 1

• These preventive approaches should be used for:

  • Female children
  • Women of childbearing potential being treated with curative intent 1
  • The dose of RhIG should be 20 mg (100 IU) to protect against 1 mL of RBCs 1
  • RhIG should be given before or soon after platelet transfusion, though it may still be efficacious if given within 72 hours 1

Clinical Implications

Alloimmunization to Rh antigens has significant clinical consequences:

• Difficulty finding compatible blood for future transfusions 3
• Risk of delayed or acute hemolytic transfusion reactions 4
• Risk of hemolytic disease of the fetus and newborn in future pregnancies
• Increased morbidity and mortality related to transfusion complications 4

Understanding the source of Rh antibody stimulation is crucial for preventing future alloimmunization and ensuring safe transfusion practices.