**Core Concept**
The development of anti-A and anti-B antibodies in infants is a critical aspect of pediatric immunology, influenced by maternal antibodies and the infant's immune system maturation.

**Why the Correct Answer is Right**
Anti-A and anti-B antibodies are typically absent in newborns due to the presence of maternal antibodies, which are transferred from the mother through the placenta or breast milk. However, these maternal antibodies gradually decline over the first six months of life, allowing the infant's immune system to produce its own antibodies against ABO blood group antigens. This process typically begins around 6 months of age, when the infant's immune system has matured sufficiently to mount an immune response against these antigens.

**Why Each Wrong Option is Incorrect**
**Option A:** Just after birth is incorrect because newborns have maternal antibodies, which provide protection against ABO blood group antigens. The infant's immune system does not yet have the capacity to produce its own antibodies.
**Option B:** 1 week after birth is incorrect because the maternal antibodies are still present in the infant's system, and the infant's immune system has not yet developed the ability to produce anti-A and anti-B antibodies.
**Option C:** 6 weeks after birth is incorrect because, although the maternal antibodies are beginning to decline, the infant's immune system still requires additional time to mature and produce its own antibodies.

**Clinical Pearl / High-Yield Fact**
It is essential for pediatricians to recognize that anti-A and anti-B antibodies may not be present in infants until around 6 months of age, as this can impact blood transfusion decisions and other medical interventions.

**✓ Correct Answer: D. After 6 months**