Haemolytic disease of the newborn can occur when
Correct answer: D. an Rh negative mother, already sensitised, carries an Rh positive fetus
- A. an Rh positive mother carries an Rh negative fetus
- B. both parents are Rh positive
- C. the mother's blood group is AB
- D. an Rh negative mother, already sensitised, carries an Rh positive fetus
Explanation
An Rh negative mother who has previously been exposed to Rh positive fetal blood makes anti D antibodies, and being IgG these cross the placenta in the next pregnancy and destroy the red cells of an Rh positive fetus. The first such pregnancy is usually safe because sensitisation happens mainly at delivery. Giving anti D immunoglobulin after each birth prevents the mother from becoming sensitised at all.
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Specific defense mechanisms provide targeted immunity through B lymphocytes, T lymphocytes, antibodies and memory cells that recognize particular antigens. Questions cover humoral and cell mediated responses, clonal selection, primary and secondary immune responses, vaccination and passive immunity, while distinguishing adaptive defenses from the immediate, non specific innate response.
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