Exploring the Human Immune System Essay

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2.4.1 Overview

The immune system can be subdivided into two systems; innate immunity and adaptive immunity. Innate immune system is made up of physical, chemical, and microbiological barriers, as well as the others elements of the immune system such as phagocytic cells (polymorphonuclear neutrophils, monocytes, macrophages), cells that release inflammatory mediator (basophils, mast cell, and eosinophils), natural killer cells, and the protein component (complement, cytokines, and acute phase proteins). Innate immune responses provide immediate host protection towards infectious agents that invade host without an immune memory and specificity (Delves and Roitt 2000b; Delves and Roitt 2000a; Parkin and Cohen 2001; Roitt et al. 2001).
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The innate responses contribute the first line of host defence and the adaptive responses take place after antigen-specific T and B cells have undergone clonal expansion and then amplify their responses by recruiting innate effector mechanisms. These provide host an intact and completely immune defence.

2.4.2 T Cells

T cells establish the basis of cellular arm of adaptive immunity. T cells originate from common lymphoid progenitor cells in bone marrow microenvironment but migrate to the thymus where they differentiate into mature T cells during T cell education. In thymus, T cell receptor (TCR) rearrangements take place whereby MHC restricted but not auto-reactive T cells are selected. During the thymic selection, double positive (CD4+CD8+) thymocytes are subjected to positive and negative selection. Positive selection ensures that only T cells with a moderate TCR affinity for self MHC are allowed to develop further whereas T cells presented with very high or low receptor affinity for self MHC will undergo apoptosis. These positively selected T cells will then enter negative selection where only T cells that recognize self antigens with low affinity are proceed to the future development. Cells that recognise self antigens with high affinity are potentially auto-reactive and will be removed by apoptosis. At the end of thymic selection, CD4+CD8+ TCRlo T cells that express TCR at high density will lose either CD4 or CD8, and

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