**Core Concept**
Rituximab is a monoclonal antibody that targets a specific antigen on the surface of B cells, leading to their depletion. This mechanism is crucial in the treatment of various B cell-related disorders, including non-Hodgkin lymphoma and rheumatoid arthritis.

**Why the Correct Answer is Right**
Rituximab specifically binds to the CD20 antigen, which is predominantly expressed on the surface of mature B cells but not on stem cells or plasma cells. This antigen is involved in the regulation of B cell activation, proliferation, and differentiation. By binding to CD20, rituximab triggers a series of intracellular signaling events that ultimately lead to B cell apoptosis. The CD20 antigen is a transmembrane protein that spans the B cell membrane four times, with the extracellular domain containing multiple glycosylation sites.

**Why Each Wrong Option is Incorrect**
**Option A:** CD 22 is another B cell surface antigen, but it is not the primary target of rituximab. CD22 is involved in B cell activation and is expressed at lower levels than CD20.
**Option B:** CD 34 is a transmembrane glycoprotein primarily expressed on hematopoietic stem cells, not B cells. It is involved in cell-cell adhesion and migration.
**Option D:** CD 55, also known as decay-accelerating factor (DAF), is a regulator of the complement system and is expressed on various cell types, including B cells. However, it is not the target of rituximab.

**Clinical Pearl / High-Yield Fact**
Rituximab is often used in combination with other immunosuppressive agents, such as cyclophosphamide and glucocorticoids, to enhance its therapeutic effects. This combination is particularly effective in treating autoimmune disorders, such as rheumatoid arthritis and lupus nephritis.

✓ Correct Answer: A. CD 20