**Core Concept**
Rituximab is a monoclonal antibody used in the treatment of various conditions, including non-Hodgkin's lymphoma and rheumatoid arthritis. It targets a specific protein on the surface of B cells, leading to their depletion.

**Why the Correct Answer is Right**
Rituximab specifically targets the CD20 antigen, a transmembrane protein expressed on the surface of mature B cells. By binding to CD20, rituximab triggers a series of cellular responses that ultimately lead to the destruction of B cells. This results in a decrease in B cell-mediated inflammation and autoimmune responses. Rituximab is a chimeric monoclonal antibody, meaning it is composed of both mouse and human protein sequences, allowing it to be used in humans while minimizing the risk of an immune response against the antibody itself.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because rituximab does not target the CD3 antigen, which is a component of the T cell receptor complex on the surface of T cells. The CD3 complex plays a crucial role in T cell activation and signaling.
**Option B:** This option is incorrect because rituximab does not target the CD4 antigen, which is a receptor on the surface of T helper cells. CD4+ T cells play a key role in cell-mediated immunity, but rituximab is not directed against this antigen.
**Option C:** This option is incorrect because rituximab does not target the CD28 antigen, which is a receptor on the surface of T cells that interacts with B7 molecules on antigen-presenting cells. While CD28 is involved in T cell activation, rituximab targets a different antigen.

**Clinical Pearl / High-Yield Fact**
Rituximab is often used in combination with other immunosuppressive agents, such as methotrexate, to treat conditions like rheumatoid arthritis. This combination therapy can lead to a significant reduction in disease activity and improvement in quality of life for affected patients.

**Correct Answer:** C. CD20