## Core Concept
The secondary immune response is a rapid and more intense response to a previously encountered antigen. It involves the activation of memory B cells and T cells, which were formed during the initial exposure to the antigen. This response is crucial for providing long-term immunity against infections.

## Why the Correct Answer is Right
The secondary immune response is primarily mediated by **memory B cells (and T cells)**, which are generated during the primary immune response. When the same antigen enters the body again, these memory cells quickly respond by proliferating and differentiating into effector cells, such as plasma cells that produce large amounts of antibodies. This rapid response results in quicker and more efficient elimination of the pathogen.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not accurately represent the cells primarily responsible for the secondary immune response.
- **Option B:** This option is incorrect as it does not specify the correct cell type involved in the secondary immune response.
- **Option C:** Although some of these cells play roles in the immune response, they are not the primary mediators of the secondary response.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the **secondary immune response** is much faster and produces more antibodies than the primary response, which is why vaccination can provide long-term immunity against certain diseases. This concept is crucial for understanding how vaccines work and why booster shots may be necessary.

## Correct Answer Line
**Correct Answer: D. Memory cells.**