Video Transcript
Which statement best describes what
happens to a memory B cell when it encounters a complementary antigen? (A) It differentiates into plasma
cells and T-helper cells. (B) It secretes large amounts of
one type of antibody. (C) It multiplies and
differentiates into plasma cells and more memory cells. (D) It only multiplies into more
memory cells. Or (E) it only multiplies and
differentiates into plasma cells.
This question asks us about memory
B cells, which are very important components of the adaptive, also called specific,
immune response. First of all, what differentiates
the adaptive immune response from the innate immune response that you are born
with? The adaptive immune response
develops over the course of one’s lifetime. And like its other name suggests,
the immune response is specific to the antigen presented, unlike innate immunity,
which is always present and reacts nonspecifically to foreign antigens in the
body. Let’s focus on the adaptive immune
system since this involves memory B cells.
These cells are created as a
long-living population after an initial, or primary, infection occurs. The primary immune response is
brought about by the cells of the adaptive immune system coming into contact with a
specific antigen for the first time. Antigens are substances that when
recognized as nonself by the immune system will trigger an immune response. These include substances such as
toxins, pollen, and molecules on the surface of pathogens or other foreign
cells.
There are two types of lymphocytes
that make up the adaptive immune response: the T cells and B cells. These become activated when they
come into contact with an antigen. Activated T cells can then
differentiate into three main types: helper T cells, which activate other immune
cells; cytotoxic T cells, which destroy infected cells; and suppressor T cells,
which close the whole attack down once the antigen has been destroyed.
B cells produce special proteins
called antibodies against the antigen. The B and T cells have receptors on
their cell surface membrane which bring about activation when they bind to the
specific antigen, a process called clonal selection. Once activated, the cells then
multiply and differentiate during clonal expansion. B cells will produce either plasma
B cells or memory B cells. The plasma cells produce the
antibodies, which will help to neutralize and reduce the effectiveness of pathogens
while recruiting other immune cells to fight the infection. After the first infection has been
controlled, the suppressor T cells will deactivate immune responses that are still
active.
However, some cells remain as
inactive memory cells that will reside in lymph nodes or bone marrow, where the
components of the blood are constantly monitored. These memory cells are able to
recognize the same antigen if it reinfects for a second time. When these memory cells come into
contact with antigens that initiated the primary immune response, a secondary immune
response begins. This is much quicker, since the
body’s immune cells are already primed to recognize and destroy the foreign
invader.
Two types of memory cells are
responsible for the secondary immune response: memory B cells and memory T
cells. Both of these immune cells are able
to recognize antigens on pathogens using receptors on their cell surface
membrane. Upon contacting the same antigen as
before, memory T cells will be activated to form helper and cytotoxic T cells, which
will then initiate further immune responses and destroy infected host cells. More memory T cells will also be
produced. Memory B cells will divide and
differentiate into more memory B cells and of course the plasma cells, which bring
about the rapid increase in antibodies seen during a secondary immune response.
Now that we have reviewed primary
and secondary immune responses as well as the cells responsible for protecting the
body, we should be able to answer our question correctly. We are asked which statement best
describes what happens to a memory B cell when it encounters a complementary
antigen. We now know that the correct answer
is (C). It multiplies and differentiates
into plasma cells and more memory cells.
