**Core Concept:**
The question is asking about the primary immune response and which component is responsible for it. The primary immune response is the initial response of the immune system to a new antigen, leading to the production of antigen-specific antibodies and the formation of memory cells.

**Why the Correct Answer is Right:**
The correct answer, **Option D: B cells**, are responsible for the primary immune response. B cells are a type of white blood cell that plays a crucial role in the immune system. When a new antigen enters the body, B cells recognize the antigen and differentiate into plasma cells that produce specific antibodies, known as immunoglobulins, that neutralize the antigen and mark it for destruction by other immune cells. Additionally, this process results in the formation of memory B cells, which provide long-lasting immunity and protect the individual from future encounters with the same antigen.

**Why Each Wrong Option is Incorrect:**
A) **Option A: T cells** are involved in the secondary immune response, which occurs after an initial encounter with an antigen. They play a role in cell-mediated immunity, not B cell activation.
B) **Option C: Macrophages** are part of the innate immune system, providing an immediate response to pathogens, but they are not directly involved in the production of antibodies or memory cells.
D) **Option D: B cells**: As mentioned earlier, B cells are responsible for producing antibodies and forming memory B cells during the primary immune response.

**Clinical Pearl / High-Yield Fact:**
Understanding the primary immune response is crucial for understanding the adaptive immune system and how the body generates immunity to new antigens. This process is essential for diagnosing and treating various diseases, infections, and allergies.