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Distinction task – Evaluate the roles of the cell-mediated and
humoral responses in pathogens
Tuberculosis (TB)
Tuberculosis (TB) is a disease which occurs due to the bacterium known as Mycobacterium
tuberculosis and mainly affects the lungs but can also influence other parts of the body –
symptoms can last for over 3 weeks and include: chest pain, coughing and tiredness. When the
TB pathogen is inhaled it encounters the first non-specific immune response in the respiratory
tract. Here, the mucus and cilia that are lined on the wall will trap the TB pathogens and either
move the pathogen upwards so it can be expelled, or it can be swallowed where it will then be
eliminated by the stomach acid. If the TB pathogen manages to get past all these non-specific
defences and enters the lungs – this can then lead to a more severe infection forming.
Once the TB bacteria does enter the lungs, the next stage of the non-specific response is to
produce antibodies that will try to attack and neutralise the TB bacteria, an example could be
how macrophages will be deployed and through the process of phagocytosis, will try to engulf
and break down the pathogen. It should be noted however, the bacteria can sometimes survive
the attack from the white blood cells – as they can adapt and even replicate within the immune
cells – leading to granulomas forming and tissue being damaged. If the non-specific response is
unable to eliminate the TB bacterium, this is the point where the specific response will be
initiated during these two responses are carried out: cell-mediated and humoral response.
During the cell-mediated response, the T cells will differentiate into either T H or TC once they are
activated by the TB antigens that are displayed on the TB bacterium's surface. Once the T H cell
recognises and binds to the specific antigens on the pathogen’s surface, the T H cells will start to
undergo clonal expansion. This is when the cell will undergo mitosis and rapidly divide and
create multiple copies of itself. These TH cells will then start to release cytokines this then results
in the activation of other immune cells (e.g., B-cells, T C cells and macrophages). The TC cells will
then identify and destroy any cells that have been infected with a pathogen – this is to prevent
the TB bacterium from reproducing and stop the spread of infection. It will do this by releasing
cytotoxins into the cell and this is what ends up killing the cell completely resulting in the
bacterium dying. The TC cell will also undergo clonal expansion which will help to increase the
number of infected cells that can be identified and destroyed.
The humoral response, on the other hand, involves the activation of B-cells (these can recognise
and bind to the specific antigens on the TB bacteria’s surface). The B-cells can then differentiate
into either plasma or memory cells. The plasma cells then undergo clonal expansion to produce
many antibodies specific to the antigen. These antibodies then bind to the surface of the TB
bacteria and act as a marker for destruction for the other components of the immune system. For
example, the antibodies can activate complement proteins – these can then attack the bacteria’s
membrane complex resulting in the lysis of the bacteria. Other antibodies can also recruit
phagocytes to perform phagocytosis to the bacteria once they have agglutinated the pathogens
making it easier for the phagocytes to kill the bacteria. Memory cells can also be differentiated
from the B-cells, and these can quickly differentiate into plasma cells if the body ever comes into
re-exposure of the same antigen. This is very important in case the person is ever infected with
TB again as the body can produce an immune response that is quicker and stronger than before,
this can help to reduce the severity of the infection and provide the body long-lasting protection
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