Video Transcript
Which of the following cells or
proteins are not involved in the humoral immune response? (A) Helper T cells, (B) cytotoxic T
cells, (C) antibodies, (D) B lymphocytes, or (E) major histocompatibility complex,
MHC, molecules.
The humoral immune response is
where antibodies that are secreted by activated B lymphocytes target pathogens and
clear them from the body by a range of mechanisms. Based on this definition, we can
immediately eliminate answer options (C) and (D), because it is clear that both
antibodies and B lymphocytes are involved in the humoral immune response.
Let’s explore the activation of B
lymphocytes in a bit more detail to determine which of the remaining answer options
is not involved. During the humoral immune response,
a B lymphocyte, or B cell, will encounter antigen-presenting cells. Each B cell possesses B cell
receptors, which recognize a different, specific antigen. Antigen-presenting cells display
their antigens on major histocompatibility complex molecules. These are known as MHC molecules
for short. Now we know that MHC molecules are
involved, we can also eliminate answer option (E).
If a B cell encounters an
antigen-presenting cell displaying its complementary antigen, the B cell receptor
will bind to the MHC–antigen complex, as we can see here. Although this interaction will
partially activate the B cell, it requires additional signals to become fully
activated. These additional signals are
usually provided by a helper T cell.
As well as recognizing antigens
with their B cell receptors, B cells can also engulf pathogens and present antigens
from the pathogen on their own MHC molecules. This MHC–antigen complex is
recognized by a helper T cell with complementary T cell receptors. Binding of the T cell receptor to
the MHC–antigen complex of the B cell causes the helper T cell to produce and
secrete cytokines. These cytokines fully activate the
B cell, which consequently differentiates into a plasma cell and undergoes clonal
expansion to produce many identical plasma cells. The plasma cells then release their
antibodies to target the pathogen and eventually clear it from the body. We have demonstrated that helper T
cells are involved. So we can eliminate answer option
(A).
The correct answer to the question
is therefore (B). Cytotoxic T cells are not involved
in the humoral immune response. They are actually part of the
cell-mediated immune response.
