Video Transcript
The figure shows a type of cell
surface protein, in green, called MHC, present on B cells. What is its function? (A) To bind with another type of
antigen on a pathogen. (B) To bind to the histamine
released from mast cells. (C) To activate phagocytosis in
neutrophils. (D) To present processed antigens
for T-helper cells to detect. Or (E) to respond to interleukins
from T-helper cells.
You may recall that MHC stands for
major histocompatibility complex. MHCs are protein complexes which
are present on the cell surface membrane of every cell that has a nucleus. And they facilitate the
presentation of antigens to other cells of the immune system.
Let’s look a bit more closely at
how this works in B cells.
As we can see in the figure, not
only do B cells possess MHC proteins on their cell surface membrane, they also have
B cell receptors. If a B cell encounters a pathogen
expressing antigens, which are complementary to its B cell receptor, it will first
engulf the pathogen by phagocytosis. Inside the B cell, the pathogen is
then digested by enzymes to break it down into the fragments we call antigens. Next, an MHC protein is assembled
around an antigen before being transported to the cell surface membrane.
It’s important to note that the
presence of an antigen stabilizes the MHC protein. So if there is no antigen present,
the MHC will not be transported to the cell surface membrane. The antigen presented by the B cell
will be detected by a T-helper cell, which will then release chemicals to fully
activate the B cell as well as other components of the immune system.
We have therefore determined that
the correct answer to the question is (D). The function of an MHC protein is
to present processed antigens for T-helper cells to detect.
