Of the five immunoglobulin classes, IgA is the main immunoglobulin of secretions from the genital, respiratory, and intestinal tracts. As a result, IgA antibody is the first line of defense against infections at the mucous membrane. It is usually an early specific antibody. Which of the following statements most accurately describes IgA?

**Core Concept**
IgA is a type of antibody that plays a crucial role in mucosal immunity by providing protection against infections at mucous membranes, such as those found in the respiratory, gastrointestinal, and genitourinary tracts. This is due to its ability to be secreted into mucosal surfaces, where it can neutralize pathogens and toxins.

**Why the Correct Answer is Right**
IgA is typically the first line of defense against infections at mucous membranes because of its ability to be secreted by mucosal epithelial cells. This is made possible by the enzyme secretory component, which is added to the antibody during its secretion, allowing it to withstand the harsh conditions of the mucosal environment. IgA can also exist in a dimeric form, which enhances its ability to neutralize pathogens.

**Why Each Wrong Option is Incorrect**
**Option A:** IgA is not the primary immunoglobulin involved in systemic immunity. While it does provide some systemic protection, its primary role is in mucosal immunity.
**Option B:** IgA is not typically associated with the activation of complement. While it can activate the alternative complement pathway, this is not its primary function.
**Option C:** IgA is not usually the first antibody to be produced in response to an infection. While it is an early specific antibody, it is often preceded by IgM in the initial stages of an immune response.

**Clinical Pearl / High-Yield Fact**
IgA deficiency is the most common primary immunodeficiency, affecting approximately 1 in 500 individuals. This can lead to recurrent infections of the respiratory, gastrointestinal, and genitourinary tracts.

**Correct Answer: D.**

✓ Correct Answer: B. IgA can be destroyed by bacterial proteases