Paper I
2022 October (Supplementary) (2010 Scheme) · 40 marks · 120 min

Question

Natural passive immunity

Q82 marksShort Notes

Answer

Natural passive immunity is the immunity conferred when pre-formed antibody is transferred naturally from one individual to another (typically from mother to offspring), without any deliberate medical intervention — the recipient’s own immune system plays no role in generating the antibody.

Mechanisms/examples:

  1. Transplacental transfer of maternal IgG — during pregnancy, IgG (the only immunoglobulin class that crosses the placenta, via active FcRn-mediated transport) is transferred from the mother’s circulation to the fetus, providing the neonate with passive protection against infections the mother is immune to, during the first few months of life until the infant’s own immune system matures.
  2. Transfer of secretory IgA via colostrum/breast milk — breastfeeding delivers secretory IgA, which provides passive mucosal protection within the infant’s gastrointestinal tract, particularly concentrated in the colostrum produced in the first few days after birth.

Characteristics: like all passive immunity, protection is immediate but short-lived, since the transferred antibody is gradually catabolised over weeks to months and no immunological memory is generated in the recipient — maternal antibody levels in the infant progressively decline, creating a period of relative vulnerability before the infant’s own active immunity (from natural exposure or vaccination) develops.

Clinical significance: explains why certain infections are rare in the first few months of life (protected by maternal antibody) but become more common thereafter as maternal antibody wanes, and underlies the timing rationale for the childhood immunization schedule.

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