What physiological circumstances lead to erythroblastosis foetalis ?

Explanation

### Step-by-Step Solution:

1. **Understanding Blood Groups**: Blood groups are classified based on two systems: ABO and Rh. The Rh factor is particularly important in pregnancy and blood transfusions.

2. **Rh Factor Explanation**: Individuals with the Rh antigen on their red blood cells (RBCs) are classified as Rh positive (Rh+), while those without it are Rh negative (Rh-).

3. **Formation of Antibodies**: If an Rh- individual is exposed to Rh+ blood, their immune system will recognize the Rh antigen as foreign and produce antibodies against it. This is a crucial point in understanding erythroblastosis fetalis.

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