Once an MHC II molecule on an APC presents an antigen:

• A. Effector cells with CD8 markers activate the APC.
• B. Effector cells with CD4 kill the APC.
• C. Effector cells with CD8 kill the APC.
• D. Effector cells with CD4 activate the APC.

Answer: (D)

Verified step by step guidance

1. Understand the role of MHC II molecules: MHC II molecules are found on antigen-presenting cells (APCs) and are responsible for presenting processed antigens to T helper cells (CD4+ T cells).
2. Recognize the function of CD4+ T cells: CD4+ T cells, also known as T helper cells, are crucial for activating and directing other immune cells. They do not directly kill cells but help in activating APCs and other immune responses.
3. Differentiate between CD4+ and CD8+ T cells: CD8+ T cells, also known as cytotoxic T cells, are primarily involved in directly killing infected or cancerous cells. They interact with MHC I molecules, not MHC II.
4. Identify the interaction between MHC II and CD4+ T cells: When an APC presents an antigen via MHC II, it is recognized by CD4+ T cells, which then become activated and help in further immune responses.
5. Conclude the correct interaction: Based on the roles of CD4+ T cells and MHC II, the correct statement is that effector cells with CD4 markers activate the APC, facilitating a broader immune response.