A naive B cell is activated when:

• A. The B cell's BCR binds to an endogenous antigen presented by an APC.
• B. The B cell is told to differentiate into a plasma cell or memory B cell by a CD4 effector cell.
• C. The B cell's BCR binds to a 'free' antigen that is not bound to an APC.
• D. When A and B occur.
• E. When B and C occur.

Answer: (E)

Verified step by step guidance

1. Understand the role of B cells in the immune system: B cells are a type of white blood cell that play a crucial role in the adaptive immune response. They are responsible for producing antibodies that target specific antigens.
2. Learn about B cell activation: B cells are activated when their B cell receptor (BCR) binds to an antigen. This can occur in different ways, such as binding to a free antigen or receiving signals from helper T cells.
3. Explore the interaction with CD4 effector cells: CD4 effector cells, also known as helper T cells, provide signals that help B cells differentiate into plasma cells or memory B cells. This interaction is crucial for the B cell's activation and subsequent function.
4. Differentiate between endogenous and free antigens: An endogenous antigen is presented by an antigen-presenting cell (APC), while a free antigen is not bound to an APC. B cells can be activated by binding to free antigens directly.
5. Combine the conditions for B cell activation: The problem states that a naive B cell is activated when it receives signals from a CD4 effector cell and binds to a free antigen. This combination of events leads to the activation and differentiation of the B cell.