CD 95 has a major role in
**Core Concept**
CD 95, also known as Fas, is a receptor that plays a crucial role in programmed cell death (apoptosis). It is a member of the tumor necrosis factor (TNF) receptor family and is expressed on the surface of various cells, including lymphocytes and epithelial cells.
**Why the Correct Answer is Right**
CD 95 interacts with its ligand, FasL, to initiate a cascade of intracellular signals that lead to apoptosis. This process is essential for maintaining immune homeostasis, eliminating self-reactive lymphocytes, and preventing tumor growth. The CD 95/Fas pathway is one of the key mechanisms by which the immune system regulates cell death and maintains tissue homeostasis.
**Why Each Wrong Option is Incorrect**
**Option A:** CD 95 is not primarily involved in cell proliferation. While it can influence cell survival, its main function is to induce apoptosis.
**Option B:** CD 28 is a co-stimulatory receptor that enhances T-cell activation, but it is not directly involved in the CD 95/Fas pathway.
**Option C:** B7-1 is a co-stimulatory molecule that interacts with CD28, but it is not related to CD 95/Fas-mediated apoptosis.
**Clinical Pearl / High-Yield Fact**
The CD 95/Fas pathway is essential for maintaining immune tolerance and preventing autoimmune diseases. Dysregulation of this pathway has been implicated in various autoimmune disorders, such as lupus and rheumatoid arthritis.
**Correct Answer: D. Apoptosis.**