**Core Concept**
The mother's milk and colostrum contain immunoglobulins that provide passive immunity to the newborn, protecting them from infections. This process is crucial for the infant's immune system development.

**Why the Correct Answer is Right**
The most common immunoglobulin secreted by the mother in milk and colostrum is IgA. IgA is produced by the mammary gland and provides mucosal immunity, preventing pathogens from entering the body through the gastrointestinal tract. This is essential for the newborn's gut health and immune system development. IgA is the primary immunoglobulin in breast milk, making up about 80-90% of the total immunoglobulins present.

**Why Each Wrong Option is Incorrect**
**Option A:** IgG is the most abundant immunoglobulin in the blood but is not the primary immunoglobulin in breast milk.
**Option B:** IgM is the first immunoglobulin produced in response to an infection and is not typically found in breast milk.
**Option C:** IgE is primarily involved in allergic reactions and is not the primary immunoglobulin in breast milk.

**Clinical Pearl / High-Yield Fact**
Breast milk contains IgA, which helps prevent infections and promotes gut health in newborns. This is why exclusive breastfeeding is recommended for the first six months of life.

**Correct Answer: C. IgA. IgA is the primary immunoglobulin in breast milk, providing mucosal immunity to newborns.**