Rh incompatibility develops when the foetus is Rh positive (Rh+) and the mother is Rh negative (Rh-). This condition occurs due to a difference in the Rh blood group system between the mother and the foetus. The Rh system is a classification of blood types based on the presence or absence of a specific antigen on the surface of red blood cells.

Explanation:
- Rh Incompatibility:
Rh incompatibility arises when the mother is Rh- and the foetus inherits the Rh antigen from the father, who is Rh+. In this case, the foetus is Rh+ while the mother is Rh-. This creates a potential problem as the mother's immune system recognizes the Rh antigen on the foetus's red blood cells as foreign and may produce antibodies against it.

- Maternal Antibody Production:
During pregnancy, if the foetal blood comes into contact with the mother's bloodstream, which can happen during events like childbirth or trauma, the mother's immune system may be sensitized to the Rh antigen. This sensitization triggers the production of Rh antibodies (specifically anti-D antibodies) by the mother's immune system.

- Subsequent Pregnancies:
If the mother becomes pregnant with another Rh+ foetus in the future, these antibodies can cross the placenta and enter the foetal bloodstream. The antibodies destroy the foetal red blood cells, leading to a condition known as hemolytic disease of the newborn (HDN) or erythroblastosis fetalis.

- Consequences of Rh Incompatibility:
The destruction of foetal red blood cells can cause various complications, including anaemia, jaundice, enlarged liver and spleen, and even severe brain damage or death in severe cases. Therefore, Rh incompatibility is a significant concern during pregnancy, especially in subsequent pregnancies.

- Prevention and Treatment:
To prevent Rh incompatibility, Rh-negative mothers who are at risk of sensitization (e.g., through previous exposure to Rh-positive blood) are given Rh immunoglobulin (RhIg) during pregnancy. RhIg prevents the mother's immune system from producing antibodies against the Rh antigen.

- Conclusion:
In conclusion, Rh incompatibility occurs when the mother is Rh- and the foetus is Rh+. This mismatch in the Rh blood group system can lead to maternal production of antibodies against the foetal Rh antigen, resulting in potential complications during subsequent pregnancies.