Okay, let's tackle this question about the EBV receptor on B cells. Hmm, I remember that Epstein-Barr virus (EBV) is a herpesvirus that infects B cells, right? The question is asking which receptor on the B cell surface it uses.

First, I need to recall the different receptors on B cells. The main ones include the B cell receptor (BCR), which is the antibody on the cell surface. But EBV probably uses a different one for entry. Wait, I think EBV uses the CD21 receptor. CD21 is also known as the complement receptor type 2 (CR2). That's because it binds to complement proteins like C3d. Oh, right! EBV's gp350 glycoprotein binds to CD21, which is why people with certain mutations in CD21 are resistant to EBV infection.

Now, let's look at the options. The correct answer is CD21. The other options might be other receptors like CD19, which is part of the BCR complex but not the EBV receptor. CD40 is involved in B cell activation through T cell interaction. And maybe CD80 or CD86, which are co-stimulatory molecules. Those aren't the EBV receptors.

Wait, the distractors might be CD19, CD40, or maybe CD80. Let me make sure. CD21 is the key here. Also, the clinical pearl is that CD21 is crucial for EBV entry, so if a patient can't make CD21, they can't get EBV. That's a high-yield fact for exams. So the correct answer should be CD21, option C.

**Core Concept**
Epstein-Barr virus (EBV) infects B cells via a specific surface receptor, which is critical for viral entry. The receptor is part of the complement system and plays a role in immune recognition.

**Why the Correct Answer is Right**
The EBV receptor on B cells is **CD21** (complement receptor type 2, CR2). EBV's glycoprotein gp350 binds to CD21, facilitating viral entry. CD21 also binds complement proteins like C3d, linking immune recognition to EBV pathogenesis. This interaction is essential for EBV to establish latency in B cells, a hallmark of infections like infectious mononucleosis and lymphomas.

**Why Each Wrong Option is Incorrect**
**Option A:** CD19 is a co-receptor in B cell antigen receptor signaling but is not the EBV receptor.
**Option B:** CD40 is a TNF receptor family member involved in B cell activation by T cells, not viral entry.
**Option D:** CD80 (B7-1) is a co-stimulatory molecule that interacts with T cell receptors, not EBV.

**Clinical Pearl / High-Yield Fact**
CD21 (CR2) is the **only known EBV receptor** on B cells. Patients with CD21 deficiency are resistant to EBV infection, highlighting its role in viral pathogenesis. Remember: EBV → gp350 → CD21 → B cell infection.

**Correct Answer: C. CD21**

✓ Correct Answer: A. CD21