Video Transcript
The figure shows an early stage in the humoral immune response. What happens next? (A) A T-helper cell releases interleukin 2. (B) The B cell secretes antibodies. (C) A T-helper cell binds to the MHC–antigen complex. Or (D) the B cell releases interleukin 2.
This figure is showing us a B cell. The B cell receptor, shown in green, has bound to an antigen on the surface of the bacterium. This signals to the B cell to engulf the pathogen and break it down. The bacterial antigens are processed and connected to a major histocompatibility complex, or MHC, molecule. This MHC–antigen complex is then moved to the cell surface of the B cell. So now what happens?
To answer this, let’s have a look at our answer choices. We can eliminate answer choice (B). In order to secrete antibodies, a B cell needs to first become activated by binding antigen. It can then differentiate to become a plasma cell, which can now secrete antibodies. Since the B cell in the figure has not yet been activated, answer choice (B) cannot be correct.
Here, we are looking at the process of how a B cell can present antigen so it can become activated. The bacterium’s antigen is presented by the MHC–antigen complex on the B cell as shown here. This can then bind to a corresponding T cell receptor, or TCR, along with the coreceptor CD4 on the T-helper cell. This causes the T-helper cell to become activated. Once activated, the process of T-helper cell expansion begins. This increases the population of these cells that would recognize this particular presented antigen. The activated T cell can then secrete a type of cytokine called interleukins that can bind to interleukin receptors on the surface of B cells. This can complete the activation of the B cell.
Now, if we look back at our answer choices, we can see that we can eliminate answer choice (D). This is because the B cell does not release interleukin. This is a function of the T-helper cell. And although one of the next steps in this process is for the T-helper cell to release interleukins, this is not the very next step in the process. So answer choice (A) is incorrect.
The correct answer is (C). Before activation of a T-helper cell and secretion of interleukins, the T-helper cell must first bind to the B cell. So the next step in the process is for a T-helper cell to bind to MHC–antigen complex.
