If an effector CD4 cell encounters an antigen presented on a MHC Class II molecule, how would it respond?

• A. Send signals to the APC trigger apoptosis in the APC.  
• B. Engulf the APC and degrade the infected cell and the intracellular pathogen.
• C. Send signals to the APC, activating the APC and triggering an immune response.
• D. All of the following are possible responses of the effector CD4 cell.

Answer: (C)

Verified step by step guidance

1. Understand the role of CD4 T cells: CD4 T cells, also known as helper T cells, play a crucial role in the immune system by assisting other cells in mounting an immune response.
2. Recognize the interaction with MHC Class II molecules: CD4 T cells specifically interact with antigens presented by MHC Class II molecules, which are typically found on professional antigen-presenting cells (APCs) like dendritic cells, macrophages, and B cells.
3. Identify the signaling process: When a CD4 T cell encounters an antigen on an MHC Class II molecule, it binds to the complex, which leads to the activation of the CD4 T cell.
4. Explore the activation of APCs: The activated CD4 T cell sends signals to the APC, often through cytokines, which enhance the APC's ability to present antigens and stimulate other immune cells.
5. Consider the immune response: The signaling from the CD4 T cell to the APC helps to amplify the immune response, leading to the activation of other immune cells such as B cells and cytotoxic T cells, which work together to eliminate the pathogen.