Which of the following immunoglobulin is responsible for passive immunity of infants?

Options

A. Ig-E, because it can cross the placenta
B. Ig-M, becasue it is heaviest immunoglobulin present in colostrum
C. Ig-A, because it is present in abundance in colostrum.
D. Ig-A, because it can cross the placenta and reaches the foetus.

Answer

To determine which immunoglobulin is responsible for passive immunity in infants, we need to understand the concept of passive immunity and the roles of different immunoglobulins. Here’s the step-by-step solution:

### Step 1: Understand Passive Immunity
Passive immunity refers to the transfer of preformed antibodies from one individual to another. In infants, this occurs primarily through maternal antibodies, which can provide immediate protection against infections.

**Hint:** Remember that passive immunity does not involve the infant's immune system producing antibodies.

### Step 2: Identify the Immunoglobulins
There are several types of immunoglobulins (Ig), including IgA, IgG, IgM, and IgE. Each has different functions and properties.

**Hint:** Familiarize yourself with the roles and characteristics of each immunoglobulin type.

### Step 3: Evaluate Each Immunoglobulin Option
1. **IgE**: This immunoglobulin is primarily involved in allergic responses and does not cross the placenta. Therefore, it is not responsible for passive immunity in infants.

**Hint:** IgE is associated with allergies, not passive immunity.

2. **IgM**: Although it is the heaviest immunoglobulin and is present in the form of a pentamer, it is not found in significant amounts in colostrum (the first milk produced by mothers). Thus, it does not contribute to passive immunity in infants.

**Hint:** Consider the presence of IgM in colostrum and its role in passive immunity.

3. **IgA**: This immunoglobulin is abundant in colostrum and is transferred to infants through breast milk. It provides passive immunity by supplying the infant with antibodies directly.

**Hint:** Focus on the role of IgA in colostrum and its importance for infants.

4. **IgG**: While IgG does cross the placenta and provides passive immunity, the question specifically asks about which immunoglobulin is responsible for passive immunity from colostrum.

**Hint:** Remember that while IgG is important, the question specifies the source as colostrum.

### Step 4: Conclusion
Based on the evaluation, the immunoglobulin responsible for passive immunity in infants, particularly through colostrum, is **IgA**.

**Final Answer:** IgA