**Core Concept:** The question is testing our understanding of the immune system and the different stages of immune responses. The mature defence mechanism refers to the adaptive immune response, which is slower but highly specific and powerful against a wide range of pathogens. The adaptive immune response involves two main components: B cells and T cells.

**Why the Correct Answer is Right:** The correct answer, B cells, are a type of white blood cell that plays a crucial role in adaptive immunity. They are responsible for producing antibodies, which are proteins that can bind to specific pathogens, marking them for destruction by other immune cells. This process allows for rapid and specific immune response against previously encountered pathogens.

**Why Each Wrong Option is Incorrect:**
A. Neutrophils: Neutrophils are part of the innate immune response and are involved in phagocytosis, a process of engulfing and destroying pathogens. They are not involved in adaptive immunity.
C. Natural Killer (NK) cells: NK cells are part of the innate immune response and are involved in killing virus-infected cells and tumor cells, but they do not participate in adaptive immunity.
D. Macrophages: Macrophages are part of the innate immune response and are involved in phagocytosis and antigen presentation. They contribute to the initiation of the adaptive immune response, but they do not produce antibodies.

**Clinical Pearl / High-Yield Fact:**
The adaptive immune response is a crucial component of the human immune system, providing long-lasting immunity against previously encountered pathogens, whereas the innate immune response is faster but less specific. This distinction between the two types of immune responses is essential for understanding immune system function and pathology, as well as treatment strategies for immune-related diseases.

**Correct Answer:** B. B cells