A 32-year-old Rh-negative multipara (2 previous vaginal deliveries) presents at 16 weeks gestation with an indirect Coombs test (ICT) titre of 1:32. Which single laboratory or clinical finding best discriminates between a **first sensitisation** (primary immune response) and **re-sensitisation** (anamnestic response) in this patient?

Explanation

## Discriminating Feature: Anamnestic Response Pattern **Key Point:** Re-sensitisation (anamnestic response) in a previously sensitised Rh-negative woman is distinguished by a **rapid, steep rise in ICT titre** with **predominantly IgG antibodies**, whereas primary sensitisation shows a slower rise with initial IgM predominance. ### Comparison Table: Primary vs Anamnestic Immune Response | Feature | Primary Sensitisation (First exposure) | Anamnestic Response (Re-sensitisation) | | --- | --- | --- | | **Timing of antibody appearance** | 4–6 weeks after exposure | Days to 1–2 weeks after re-exposure | | **Initial antibody class** | IgM (early), then IgG (weeks 4–8) | IgG (immediate, from memory B cells) | | **ICT titre pattern** | Slow, gradual rise | Rapid, steep rise (often doubling every 2–4 weeks) | | **Peak titre** | Lower (typically <1:16) | Higher (often >1:32 by 2nd trimester) | | **Clinical severity** | Mild in first pregnancy | Severe in subsequent pregnancies | | **Fetal risk** | Low in first sensitised pregnancy | High; risk increases with each pregnancy | **High-Yield:** The **anamnestic response** is the hallmark of re-sensitisation. Memory B cells from previous exposure rapidly differentiate into plasma cells, producing large amounts of IgG antibodies without the lag phase seen in primary response. This explains why: - 1st sensitised pregnancy: usually mild HDN - 2nd sensitised pregnancy: moderate to severe HDN - 3rd+ sensitised pregnancy: severe HDN, hydrops, IUFD **Mnemonic: "RAPID-IgG"** — Re-sensitisation = Anamnestic response = Prompt IgG rise (Immediate, Dramatic titre increase, IgG-mediated). ### Clinical Interpretation in This Case This patient is a **multipara with 2 previous deliveries**. An ICT titre of 1:32 at 16 weeks strongly suggests **anamnestic re-sensitisation** from prior deliveries. The rapid rise in IgG-mediated antibodies indicates memory B cell activation and predicts severe fetal haemolysis. Serial monitoring and early intervention (e.g., intrauterine transfusion if MCA-PSV elevated) are warranted. **Clinical Pearl:** In a previously sensitised woman, even a "low" ICT titre at booking (e.g., 1:8) can rapidly escalate to 1:32–1:64 within weeks due to anamnestic response. This is why **serial ICT measurements** (every 2–4 weeks) are critical in sensitised pregnancies.