**Core Concept:**
Cellular immunity is a part of adaptive immunity that involves the activation, proliferation, and differentiation of lymphocytes, particularly T cells and Natural Killer (NK) cells, to eliminate pathogens, infected cells, and tumor cells. It is a crucial defense mechanism against intracellular pathogens, viruses, and certain types of cancer.

**Why the Correct Answer is Right:**
The correct answer is B, **Antigen-Presenting Cells (APCs)** like dendritic cells, macrophages, and B cells. APCs capture, process, and present antigens (foreign substances) to T cells via major histocompatibility complex (MHC) molecules. This interaction triggers the activation, proliferation, and differentiation of T cells, leading to the development of a specific immune response against the presented antigen.

**Why Each Wrong Option is Incorrect:**
A. **Lymphocytes**: While lymphocytes are essential components of the adaptive immune system, they do not directly induce cellular immunity. Instead, they participate in the activation and regulation of cellular immunity after being activated by APCs.

C. **T-cells**: While T cells play a crucial role in cellular immunity, they require activation and stimulation by APCs before they can perform their immunological functions.

D. **Natural Killer (NK) cells**: NK cells are part of the innate immune system and contribute to the immune response against virus infections and tumor cells. However, they do not initiate cellular immunity but rather enhance and modulate the response initiated by APCs and T cells.

**Clinical Pearl / High-Yield Fact:**
Understanding the role of APCs in initiating cellular immunity is crucial for interpreting immune responses and selecting appropriate immunotherapeutic strategies in various clinical scenarios, such as autoimmune diseases, transplantation, and cancer immunotherapy.