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Leukocytes is another term for a White Blood Cell, these are cells that are very important for the immune system and its defence against pathogens and other harmful substances. Leucocytes circulate throughout the blood stream and the lymphatic system, accumulating at limhcatic node and a sight of infection or as part of the clotting process: (To prevent subsequent infection from an open wound.) All Leukocytes are generated from Hematopoietic stem cells;  these are Stem cells that differentiate and specialize into various kind of blood cell including erythrocytes. (Red Blood Cells).

Typically there are 5 variants of leucoytes: spanning from ones who are involved in the none specific primary immune response, to highly specialised T cells and B cells that are specifically complementary to a specific antigen of a pathogen. Monocytes are the largest type of cell. There responsibility lies within the innate immune defence, they act rapidly in response to an infection; differentiating into different cells, such as macrophages. These cells then will proceed to engulf the pathogen and attempt to break it down using hydrolytic enzymes. Macrophages also may display the pathogens own antigens on its cell surface, in order to trigger a specific immune response to the pathogen.

Eosinophils are another variant of the White blood cell (WBC). These cells are found in the blood. These cells are still part of innate immunity acting on generic targets. As well as defence from pathogens; these cells also play a role in the management of the allergic inflammatory response. Their main attribute are granules stored in the cell that contains a wide variety or chemical mediators and inflammatory material.  These granules once derogated release these mediators which kills invading pathogens- incidentally it also  harmful to host tissue. Problems with these cells can lead to inappropriate immune responses; which can be potentially lethal.

The next specialism of Leukocyte is the Basophil. These have a more specific role in the immune response, mediating an allergic response and managing the release of histamine to control inflammation. These cells do also contribute to an immune response against a pathogen but their role is minimal. Basophils also have a role in humoral immunity; as they can interact with IgE (Immunoglobulin E) this molecule also plays a role in the bodies' response to an allergic reaction.

The next Leukocyte is the most abundant immune cell in the body. The Neutrophil. Initially it is responsible for innate immunity, usually these cells are responsible for phagocytosis; the bodies initial reaction to a foreign pathogen. They are also crucial in the response to an open wound; accumulating at the area of injury to prevent bacterial infections.

Lymphocytes are perhaps the most specialised of the leukocytes. There are 3 common lymphocytes: the cytotoxic T cell, The memory B cell and the Natural Killer or (NK) cell.  All of these are specific to one infection due to how they function. Their roles involve all kinds of immunity. Cell mediated, humoral and adaptive.  T cells produced in the Thymus gland are the specialist response in cell mediated immunity. These cells contain antibodies; these are complementary to the pathogens surface antigens. Thus there is an interaction between the two leading to breaks in the capsule and cell wall, this causes the pathogen to lyse. Helper T cells and Cytoxic T cells, produce molecules such as enzymes that also trigger the degradation of a pathogen and any infected cell. B cells that are produced in the Bone marrow, have a similar effect. Both cells can specialise to form memory cells. These cells contain copies of the antibodies that were complementary to the pathogens antigens. Thus the next infection is a much more rapid response. The way this works is also the basis for Vaccinations. 

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