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This is the system that protects the body from harmful substances/microorganisms, they are like the soldiers of the body because they defend the body and kill the invaders. Immunity given by the immune system is the protection of the body from infections and agents that cause disease (Latha, 2012). This system is the most complicated system in the body because it is made up of various organs, tissues and cells apart from the nervous system. Without this system present in the body, the cells would constantly be invaded by different kinds of harmful microorganisms or substances.
The blood and lymph systems are very important in coordinating an attack against infectious organism and other invaders. Given that the biological kingdom has many different pathogens, the immune response to such organisms are not always similar. The response of the immune system depends on different variables such as mode of entry, type of pathogen and behaviour of pathogen (Latha, 2012). The main responsibilities of the immune system is to; neutralise invaders like bacteria, viruses, fungi, and protozoans, and remove them; recognise their presence in the body immediately they invade through cytokines and toll – like receptors; Fighting against cancerous cells using killer T cells.
The immune system comes into action when non-self-substances enter the cells. These substances are the proteins on the surface of bacteria, viruses, and fungi, and they are called Antigens. When these antigens bind to the receptor sites on the surface of the immune system cells, a combination of processes are put into place. The immune system has the ability to store memories and can remember them when known pathogens attack again; this enables the process to be faster. The body cells also have surface proteins but the immune system cells have been programmed to distinguish between its own surface proteins and foreign ones. In the case that the immune system attacks self-cells, viewing it as non self, it is called an autoimmune reaction. This reaction can be triggered by a number of things for example, a combination of infections.
The reason why the immune system can function properly is due to self-tolerance; its ability to differentiate between self-reactive and non-self-reactive lymphocytes because the body is not supposed to work against its own healthy cells. This ability of the immune system to tolerate it cells helps it to pin point and eliminate the lymphocytes that have the potential to be self-reactive. Even with this mechanism, some defective (self-reactive lymphocytes) still escape, exit the thymus, and are able to be activated to bring about autoimmune diseases.
The immune system is also referred to as lymphoid system because lymphoid tissue and lymphoid organs form the core of the immune system. The organs of the immune system are divided into primary lymphoid organs and secondary lymphoid organs.
These organs are also known as central lymphoid organs. In these organs, differentiation, proliferation and maturation of pluripotent cells into immuno competent cells occur. The central lymphoid organs are thymus and bone marrow. The thymus is where T cells are produced after the differentiation and maturation of hematopoietic stem cells. For the B cells differentiation and maturation occur in the bone marrow. The mature T and B cells in the primary lymphoid organs do not show any response because they have not been exposed to antigens since presence of antigens is necessary for an immune response.
The thymus is a flattened bi-lobed organ with lymph epithelial tissue above the heart and beneath the sternum. Source lymphocytes of hematopoietic stem cells move from the bone marrow to the thymus via the blood stream in other to undergo differentiation and maturation into immuno competent cells. Each of the thymus lobes is organised into an outer cortex and an inner medulla. The peripheral cortex of each lobe is very populated with differentiated but immature T cells or thymocytes while the medulla is filled with mature lymphocytes that are loosely arranged. Given that there are mature T cells in the cortex and medulla, the order in which maturation occurs is not totally understood.
In the thymus the lymphocytes increase quickly by majority of them die without ever functioning, leaving about 5% of the cells for maturation. This might be because they failed to react with antigens and/or react with self-antigen, and so were deleted during the selection process. Thymus epithelial cells secretes a hormone called thymosin which helps in the growth of T cells by stimulating their development. The remaining T cells that did not die move out of the thymus into the peripheral blood and lymph, and colonize the secondary or peripheral lymphoid organs that the thymus is dependent on. When they interact with an antigen they become active and give cellular immunity. The structural frame work of the thymus is a three-dimensional structure which is composed of inter digitating dendritic cells, stomata cells, epithelial cells and macrophages. The epithelial cells have long extensions which surrounds as much as 50 thymocytes in the cortex and also the blood vessels, and lymph spaces of the medulla and cortex. The function of the membrane of epithelial cells is probably to prevent macromolecules, which includes antigens, of the blood to come in contact with the T cells in the lymphocytes. Therefore, the epithelial cells are referred to as nurse cells. The presence of these nurse cells is most likely the reason why there are no immune reactions in the thymus.
In childhood, thymus activity reaches its peak but by puberty it gets to its largest size and later decreases in size (known as atrophy), an in old age, it becomes very small. Therefore, immunity declines as one gets to old age. The thymus reduces due to increase in total fat of the organ and decrease in the cortical and medulla cells size. Given that T cells play an important role in immunity, it is essential that the thymus develops completely. Aplasia of thymus is the failure of the thymus to develop and it results in defects in immune reactions involving T cells. In order to deal with problems that may arise in the absence of thymus, the organ can be replaced or supplements of the thymosin can be used.
These organs are also known as peripheral lymphoid organs and where the mature lymphocytes stay and function in response to antigenic stimuli. These mature lymphocytes are cells from the primary lymphoid organs which are competent enough to protect the body from foreign antigens. It is in these organs that immune competent cells interact with foreign agents and carry out immune response. Antibodies from activated B cells are produced here, as well as activation of cytotoxic T lymphocytes. The peripheral organs may either be encapsulated to have a specific shape or in the form of diffused tissues. The encapsulated organs are spleen, lymph nodes, tonsils and adenoids. The mucosa associated lymphoid tissue which is distributed all over the body such as the alimentary and respiratory tracts are examples of the diffused secondary lymphoid organs. The secondary lymphoid organs are usually under developed at birth and shows progressive growth with age unlike the primary lymphoid organs which decreases with age.
As mentioned above, the spleen is an encapsulated secondary lymphoid organ. Large, red, and oval shaped, the spleen is located in the left side of the abdominal cavity below the pancreas. This organ responds to systemic infections because of its contact with the main arterial circulation. A histological study of the spleen shows the penetration of connective tissue from capsule into the spleen to divide it into centrally connected compartments. There are two different regions in each compartment; the red pulp and the white pulp. The red pulp region is non-lymphoid and it consists of a network of sinusoids filled with red blood cells (RBCs) and macrophages. It is in this region that destruction of defective and old RBCs occurs. It is also the site for blood cells in early development stages. In adult’s spleen produces RBCs with the bone marrow in order to face crisis when there is an emergency. The white pulp region is lymphoid, and in this medium, a major part of the lymphoid tissue arranges around a central arteriole which forms a periarterial lymphatic sheath (PALS). Since Malphigi was the first to describe the arrangement of lymphocytes, it is called malphigian body. In the malphigian body, next to the PALS are the B cells, which are organized in the form of lymphoid follicles, in the marginal zone. The T lymphocytes are distributed with the dendrite cells and present around the arteriole are the interdigitating cells, so that when the blood borne antigens enter the malphigian body of the spleen through the artery, the interdigitating dendritic cells (antigen presenting accessory cells) would capture the pathogens present and present them to the T cell lymphocytes. This activates the T cells which in turn activates the B cells.
This is another encapsulated secondary lymphoid organ, which is bean shaped and complex. The reticular network filled with lymphocytes, macrophages and dendritic cells are enclosed with connective tissue capsule. They are clustered at the junction of lymphatic vessels and they filter lymph. Pathogens that enter through the gastrointestinal tract or respiratory tract would have to pass through regional lymph nodes where they are either effectively trapped for phagocytosis due to the presence of macrophages in the lymph nodes or initiate an immunological response from T and B cells. The point in the lymph node where blood enters and exits is known as the Hilus. There are several afferent lymphatic ducts through which lymph enters this organ but after it has been filtered, it exits through a single duct. The internal environment of the lymph nodes has three different areas, histologically; the cortex, the para cortex and the medulla. The cortex region of the organ contains B cells, macrophages, and follicular dendritic cells arranged as primary follicles. When there is an interaction with antigens, the primary follicles would enlarge into secondary follicles with a germinal center. The development of two germinal centre means that the B cells are actively differentiating and preparing to eliminate the antigen. These secondary follicles with germinal centers appear more during secondary immune response than in primary immune response. The paracortex is densely populated with T cells and interdigitating dendritic cells.
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