## **Core Concept**
The question tests the understanding of the primary immune response and the types of antibodies produced. When an antigen is introduced to the body for the first time, it triggers a primary immune response. This response is characterized by the production of specific classes of antibodies.

## **Why the Correct Answer is Right**
In a primary immune response, the initial antibodies produced are primarily of the **IgM** class. IgM is the first antibody to be made by the body to fight a new infection and is very effective in activating the complement system. The production of IgM is immediate, but it has a relatively short half-life.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because, although IgG is a crucial antibody class produced in response to many antigens, it is predominantly associated with secondary immune responses. IgG production starts later and persists longer than IgM.
- **Option B:** This option might seem plausible because IgA is an important antibody class, especially in mucosal immunity. However, it is not the primary class of antibodies produced in response to a systemic antigen like ovalbumin administered for the first time.
- **Option C:** This option is incorrect because IgE is primarily involved in allergic reactions and protection against parasites. It is not the primary antibody class produced in response to ovalbumin when administered for the first time.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that in the primary immune response, **IgM** is the first line of defense and is produced early, whereas **IgG** becomes predominant later and provides long-term immunity. This distinction is crucial for understanding immune responses and vaccine development.

## **Correct Answer:** . IgM.