Explain passive immunity across the placenta
Identify transferred antibodies and distinguish temporary protection from recipient immune memory.
Extended only (Supplement); Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
Before you begin
- Recall active immunity and antibody proteins.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What crosses in the required passive example?
What you will learn
- Ready-made antibodies come from another individual
- The passive transfer does not create memory
Ready-made antibodies come from another individual
Passive immunity is short-term defence supplied by antibodies acquired from another individual. In the placenta example, some maternal antibodies cross the placenta and reach the developing infant. The mother produced these proteins; their transfer gives the recipient protection without requiring the recipient to make those particular antibodies through the transfer itself.
The placenta is the exchange interface, not a route that mixes all maternal and fetal blood together. The required explanation is transfer of antibodies, not transfer of maternal lymphocytes, memory cells or the pathogen itself. Protection remains specific to targets recognised by the transferred antibodies.
The passive transfer does not create memory
Memory cells are not produced by passive antibody transfer. The acquired proteins can be broken down over time, so protection declines rather than being renewed by an induced memory response. This explains the syllabus description of passive protection as short term without assigning a fixed universal number of days.
The recipient can still develop active immune responses to other exposures; stating that the passive transfer produces no memory does not mean the infant is permanently incapable of immune memory. Classify the described process by where the antibodies were produced and whether the recipient’s own response was stimulated.
Pause and explain
Why is protection short term in the passive model?
Worked example
Maternal antibodies cross the placenta without stimulating a recipient response. Identify the immunity and memory result.
Show the worked solution
- The proteins are acquired from another individual.
- The transfer itself does not create memory cells.
Answer Passive immunity; no memory cells are produced by this antibody transfer.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State the antibody source and route in placenta-mediated passive immunity.
Give me a hint
Identify producer and exchange interface.
Compare my reasoning
- The mother produced antibodies.
- Some pass across the placenta to the infant.
Maternal antibodies are acquired by the infant across the placenta.
Look for these in your work
- Name maternal source
- Name placenta route
Explain why this passive transfer does not establish longer-term immune memory.
Give me a hint
Transferred protein is not a stimulated memory response.
Compare my reasoning
- The transfer itself produces no memory cells.
- Acquired antibodies can be broken down over time.
No memory cells are produced by passive antibody transfer, so acquired-antibody protection declines as those proteins are lost.
Look for these in your work
- Name lack of memory
- Link antibody loss to duration
Correct “passive immunity means the infant can never make memory cells”.
Give me a hint
Limit the claim to the described process.
Compare my reasoning
- Passive transfer itself induces no memory cells.
- Separate active responses can still produce memory.
Only the passive transfer lacks memory-cell production; this does not prevent the recipient from developing active immune memory through other exposures.
Look for these in your work
- Keep the process boundary
- Avoid a permanent incapacity claim
Common mistakes
- Saying all maternal blood mixes with fetal blood.
- Treating acquired antibodies as acquired memory cells.
What can you explain now?
Why is protection short term in the passive model?
Compare with the explanation
Transferred antibodies decline without induced memory cells
The proteins are not a self-renewing response.
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Make it stick
Explain source, route and duration of passive placental protection.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.