Video Transcript
The figure shows a B lymphocyte
with antibody, immunoglobulin, receptors on its surface membrane. Which statement is not true? (A) The B cell antigen receptor has
a similar structure to a soluble antibody. (B) Each B cell will have a
different type of antibody receptor on its surface. (C) The B cell can bind to
extracellular and intracellular antigens. (D) Each B cell has only one type
of antibody receptor that binds to one specific antigen. (E) The B cell antibody receptor is
complementary to an antigen.
Let’s start by discussing the B
lymphocyte in the given image. B lymphocytes have a special
genetic makeup. During the formation of B cells
from stem cells via precursor lymphocytes, the genes which code for the different
parts of antibody receptors are shuffled by some type of genetic recombination. As a consequence, each precursor
gives rise to a B cell with a unique type of antibody receptor. Each B cell expresses a unique
model of antibody receptors that is different from other B cells. We can therefore rule out answer
(B), as we know this is a correct statement.
Now, let’s look at how the B
lymphocytes fight their pathogens. Pathogens release substances or
contain specific proteins on their cell surface which differ from molecules
contained in our own body. These foreign molecules are called
antigens. They can be recognized by a
specific B cell. Indeed, thanks to the vast variety
of antibody receptors that are produced, there is a high probability that one of
them will be complementary to an antigen. Then, the antigen binds to one
specific antibody receptor at the surface of a B cell. Therefore, we agree with statements
(D) and (E). We can rule out these two answers
because they are correct.
When an antigen binds to an
antibody receptor on a B cell, the B cell becomes activated and clones itself. These clones can then differentiate
to become plasma cells which release antibodies. These antibodies are structurally
very similar to the antibody receptors. But they are free-flowing and not
membrane-bound. These soluble antibodies can attach
to the specific antigen of the pathogen and immobilize it. They help us fight the infection
caused by the pathogen. So, with this information, we can
rule out option (A) as well, since it’s correct to say that the B cell antigen
receptor has a similar structure to a soluble antibody.
Now, let’s examine our remaining
option, which is (C). Is it wrong to say that the B cell
can bind to extracellular and intracellular antigens? B cells are cells involved in the
specific immune response, also called acquired or sometimes adaptive immune
response. This immune response is often
divided into humoral and cell-mediated immunity. This confirms that statement (C) is
not true. The B cell can bind to
extracellular antigens. But they can’t bind to
intracellular antigens. The answer to the question is (C):
the B cells can bind to extracellular and intracellular antigens.
