Why do children cured by enzyme-replacement therapy for adenosine deaminase dediciency need periodic tratment ?

Explanation

**Step-by-Step Solution:**

1. **Understanding Adenosine Deaminase Deficiency (ADA Deficiency)**:
- Adenosine deaminase (ADA) deficiency is a genetic disorder that affects the immune system. It leads to the accumulation of toxic metabolites, which can severely impair immune function.

2. **Enzyme Replacement Therapy (ERT)**:
- Enzyme replacement therapy is a treatment designed to provide the missing or deficient enzyme—in this case, ADA—to patients. This therapy helps to alleviate the symptoms and improve the immune response.

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