**Core Concept:** Proteoglycans are complex macromolecules that play a crucial role in maintaining the structure and function of tissues, particularly in connective, epithelial, and neural tissues. They are involved in cell adhesion, growth, and differentiation, as well as tissue organization and mechanical support.

**Why the Correct Answer is Right:**

Proteoglycans are composed of a protein core (proteins) to which glycosaminoglycans (GAGs) are covalently attached. The core proteins are responsible for tissue-specific functions, while GAGs provide the bulk and charge of the proteoglycan. The correct answer states that proteoglycans are involved in the regulation of cell adhesion, as they interact with extracellular matrix proteins and growth factors, thereby influencing cell behavior.

**Why Each Wrong Option is Incorrect:**

A. False: Proteoglycans are not involved in the regulation of cell proliferation. They primarily affect cell adhesion and differentiation, rather than proliferation.
B. False: Proteoglycans do not solely regulate the immune system. Their primary function is in tissue organization and mechanical support.
C. False: Proteoglycans do not exclusively function in cell motility. They contribute to cell adhesion, differentiation, and tissue organization.
D. False: Proteoglycans are not solely involved in cell migration. Their role extends to cell adhesion, differentiation, and tissue organization.

**Clinical Pearl:**

Proteoglycans are essential components of various tissues and play a critical role in maintaining tissue integrity and function. Understanding their roles in cell adhesion, differentiation, and tissue organization is crucial for understanding tissue repair, regeneration, and pathological conditions like arthritis.