## **Core Concept**
Ofatumumab is a monoclonal antibody used in the treatment of certain types of cancer and autoimmune diseases. It specifically targets a protein on the surface of B cells. The mechanism of action involves the depletion of B cells.

## **Why the Correct Answer is Right**
Ofatumumab targets **CD20**, a protein found on the surface of B cells. By binding to CD20, ofatumumab induces B cell depletion through mechanisms such as complement-dependent cytotoxicity (CDC), antibody-dependent cellular cytotoxicity (ADCC), and apoptosis. This makes option **B. CD20** the correct answer.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Not specified, but assuming it's another protein or antigen, it's incorrect because it doesn't accurately represent the target of ofatumumab.
- **Option C:** Not specified, but if it's another antigen or protein not related to B cell targeting by ofatumumab, it's incorrect for the same reason.
- **Option D:** Similarly, if this option does not correspond to CD20, it is incorrect.

## **Clinical Pearl / High-Yield Fact**
Ofatumumab is particularly noted for its use in treating chronic lymphocytic leukemia (CLL) and multiple sclerosis. A key point to remember is that drugs targeting CD20, like ofatumumab and rituximab, are effective in depleting B cells, which play a significant role in autoimmune diseases and certain malignancies.

## **Correct Answer:** B. CD20