Video Transcript
If a person is infected with the
bacteria that causes pulmonary tuberculosis when they are younger, they are highly
likely to not be affected by tuberculosis again when they are older. Why is that? (A) The person’s barrier defenses
will be stronger after the first infection and prevent future pathogen entry. (B) The plasma cells produced after
B cell activation will destroy all body cells that are susceptible to being
infected. (C) Memory B cells are produced
after the first infection, and these initiate a rapid immune response if a person is
infected a second time. Or (D) this is incorrect; if they
catch pulmonary tuberculosis once, they are highly likely to be affected by it
again.
Let’s remove the answer options for
now and remind ourselves what happens when the body is reinfected with a
pathogen.
When the body is first infected
with a pathogen, like the bacterium which causes pulmonary tuberculosis, the
nonspecific immune response is immediately activated, followed about four to seven
days later by the specific immune response. Part of the specific immune
response involves the activation of B cells. When a B cell encounters a cell
presenting its complementary antigen, its B cell receptor binds to the antigen. This activates the B cell, which
consequently undergoes clonal expansion.
As shown by the diagram, clonal
expansion is where many identical clones of the activated B cell are produced. The majority of these clones will
differentiate into plasma cells, which secrete antibodies that circulate in the
blood and fight off the infection. However, a small proportion of them
will differentiate into memory B cells, which can remain in the lymph nodes for
several decades, sometimes 40 or even 60 years.
If the body is reinfected with the
same pathogen again in the future, the memory B cells will rapidly differentiate
into plasma cells and many more antibodies will be secreted. This secondary immune response will
be much faster and more powerful than the primary response that followed the first
infection. In fact, the secondary response can
be so effective that the person will often not even know they have been infected a
second time.
We can therefore conclude that the
correct answer to the question is (C). Memory B cells are produced after
the first infection, and these initiate a rapid immune response if a person is
infected a second time.
