**Question:** Primary immune response is mediated by -

A. B cells
B. T cells
C. Mast cells
D. Eosinophils

**Core Concept:**
The primary immune response refers to the initial activation and development of adaptive immune cells, specifically B cells and T cells, in response to a new antigen. Both B cells and T cells play crucial roles in the immune system's ability to recognize and neutralize pathogens.

**Why the Correct Answer is Right:**
In the primary immune response, B cells (B lymphocytes) are responsible for producing antibodies against the specific antigen. When a new antigen enters the body, B cells differentiate into plasma cells and memory B cells. Memory B cells help in a secondary immune response, which is faster and more effective than the primary response. Additionally, T cells (T lymphocytes) are involved in coordinating the immune response, including activating B cells and other immune cells.

**Why Each Wrong Option is Incorrect:**
A. Mast cells (MC) are involved in the immediate hypersensitivity reactions, specifically type 1 and 2 reactions, and are not directly involved in the primary immune response.
B. Eosinophils primarily play a role in immune response against parasites and helminths, not in the primary immune response against new antigens.
C. Memory T cells and memory B cells are involved in the secondary immune response, not the primary immune response.

**Clinical Pearl:**
Understanding the primary immune response is essential for medical students and practitioners alike, as it forms the basis for a robust adaptive immune response against pathogens and, consequently, contributes to preventing infections and diseases. The role of B cells and T cells in the primary immune response helps in recognizing and neutralizing new antigens, contributing to the body's overall defense mechanism.