**Core Concept**
The primary immune response, also known as the first antibody response, is a complex process involving the activation of various immune cells and the production of antibodies. This response is crucial for protecting the body against infections and involves the collaboration of different immune cells, including B cells and T cells.

**Why the Correct Answer is Right**
The first antibody response is primarily mediated by B cells, which recognize antigens through their surface-bound antibodies (BCRs). Upon antigen recognition, B cells undergo activation, proliferation, and differentiation into antibody-secreting plasma cells. This process is facilitated by the help of T cells, particularly T helper 2 (Th2) cells, which secrete cytokines that promote B cell activation and antibody production. The resulting antibodies are primarily of the IgM isotype, which is the first line of defense against infections.

**Why Each Wrong Option is Incorrect**
**Option A:** This answer choice is incorrect because it does not accurately describe the primary mediator of the first antibody response. While T cells play a crucial role in the immune response, they are not the primary mediators of the first antibody response.
**Option B:** This answer choice is incorrect because it is too broad and does not specifically describe the primary mediator of the first antibody response. The immune system involves multiple cell types and mechanisms, but B cells are the primary mediators of the first antibody response.
**Option C:** This answer choice is incorrect because it is not relevant to the first antibody response. While memory cells are important for long-term immunity, they are not the primary mediators of the first antibody response.

**Clinical Pearl / High-Yield Fact**
The first antibody response is a complex process involving the collaboration of multiple immune cells, including B cells and T cells. Understanding the primary mediator of this response is crucial for developing effective vaccines and immunotherapies.

**Correct Answer: A. B cells.**