THE ADAPTIVE IMMUNE SYSTEM
Unlike the innate system, the function of the adaptive immune system is to destroy pathogens and mitigate any production / spread created once breached the innate lines of defences. Due to their destructive nature, the adaptive immune system relies on specific identification of the pathogen in order to only respond to molecules that are foreign to the host, and not to the molecules of the host itself. Furthermore, the adaptive immune system memorises and responds differently to each pathogen in order to most effectively eliminate all traces from the host in the proceeding encounters it may have.
Lymphocytes that are involved in the adaptive immune system
Lymphocytes that play a significant role in the adaptive immune system can be categorised under two distinctive groups;
B cells, which are produced in the bone marrow, and are responsible for antibody mediated immunity; the specific immune response that requires B cells to attack and remove pathogens that are outside of the cells
T cells, which are similarly produced in the bone morrow, however, are matured in the thymus, and are responsible for cell mediated immunity; the specific immune response that utilizes T cells to neutralize cells that have been infected with viruses and certain bacteria.
Both types of cells have three stages that they can be in:
-Naïve stage, where cells mature within their respective tissues but have not been exposed to an antigen
-Effector stage, where cells have encountered a specific antigen and have become an active participant in the removal process
-Memory stage, where cells continue to circulate after the destruction of the pathogen. If the same pathogen is to be encountered, the memory cells quickly initiate an immune response.
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Given that they are the only cells in the organism able to recognize and respond specifically to each antigenic epitope, the T and B cells play a significant role in the adaptive immune system, and ultimately preserving the health of the organism.
The role of b cells in adaptive immunity
B cells are considered one of the most significant components of the adaptive immune system, due to its ability to produce antibodies and present antigens to T cells. Once produced and matured in the bone marrow, B cells can be differentiated into the following forms:
- Plasma (effector cells) which allows them to produce large quantities of antibodies against specific antigens, thus ensuring the identification of all present pathogens, minimising the risk of not locating, and thus not identifying a particular pathogen. Effector B cells respond to signals from T cells during infection and continue to produce antibodies until the infection is controlled.Â
- B Memory cells enable the efficient identification and initialisation of the immune response. If the host is re-exposed to the same antigen, these cells rapidly multiply with assistance from T cells. This produces more cells capable of secreting specific antibodies to the pathogen, and thus rapidly supressing the infection before significant damage can be done.
- The previous differential forms of B cells have been dependant on T cells for some aspects of their role, however a small percentage of B cells can differentiate into T-independent B cells, which are particularly important for dealing with encapsulated bacteria, which have a specialised outer layer as opposed to the general protein-based layer, resultingly allowing them to evade T cells. T-independent B cells can recognise these layers and produce antibodies without the assistance of T cells.
The role of T cells in adaptive immunity
T cells are also one of the most significant components in the adaptive immune system, due to their various roles and responsibilities to ensure the adequate immune response is carried out. T cells not only assist in the activation of B cells, allowing them to secrete antibodies and macrophages, they also help activate cytotoxic T cells to kill infected target cells, and contribute to the maturation of B cells into plasma / memory cells, alongside the production of appropriate antibodies.
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T cells differentiate into various forms dependant on which recognizable APC (antigen presenting cell) it encounters. There are three types of signals: TCR, BCR, and cytokine signals. If a cell receives all three signals, it will mature into an effector cell. However, if a cell only receives one of the TCR or BCR signals, the cell will become useless.
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Thus, various forms are created each with their own unique responsibilities in the immune response. Such forms are as follows;
- Effector cells, which carry out the functions of an immune response. They can be cytotoxic, helper, and regulatory T cells:
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- Cytotoxic T Cells, which kill toxic/target cells. Upon recognition, their purpose becomes the removal of infected cells and bacteria via apoptosis; where a cell’s organelles are destroyed, causing it to die from the inside out.
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- Similarly to Cytotoxic T cells, Helper T cells also aid in the removal of toxic/target cells. However, T Helper cells utilise a wider range of functions; upon activation, they possess the ability to differentiate into cell subtypes when presented with antigens, and also multiply and secrete cytokines that summon macrophages and cytotoxic T cells to the infected site.
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-Regulatory T cells are responsible for stopping an autoimmune response once the threat has been eliminated to avoid an influx of remaining Cytotoxic/Helper cells that could potentially attack healthy cells. This is due to the fact that after the removal of a pathogen, the Cytotoxic/Helper T cells that bound to it are no longer needed.
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