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This system can recognize and eliminate specific foreign microbes and molecules that enter to the body. Adaptive immune system shows four characteristic features
Antibodies can differentiate between two protein molecules that only differ in one single amino acid. The immune system is capable for generating diversity in its recognizing molecules.
This ability helps for pattern recognition molecules of the innate system. Once the immune system has recognized and responded to an antigen, it exhibits immunologic memory then for the second time it induce a high state of reactivity. Because of this property, the immune system can confer lifelong immunity to many infections after initial encounter. The immune system responds only to foreign antigens, indicating it is capable of Self-nonself recognition.
The cells of the adaptive immune system are special types of leukocytes that are known as lymphocytes. B cells and T cells are the major type of lymphocytes, these are derived from the hematopoietic stem cells in bone marrow. Both lymphocytes contain receptor molecules that recognize specific antigen. The lymphocytes leave the bone marrow after maturation and circulate in the blood stream and lymphoid organs as it contains receptors.
B lymphocytes mature in the bone marrow and on release it exhibits antigen-receptors on its membrane. The antigen-receptors are a membrane antibody molecule. These antibodies are glycoproteins that consist of two identical polypeptides. When the antigen matches to the antibody present on the membrane, it leads to division and the progenies gets differentiated into memory B cell and effector B cells called as plasma cells. Memory B cells have more life span than that of the naïve cell; they also express antibodies similar to that of their parent B cell. In plasma cells they produce antibodies in the form of secretion which have no membrane bound antibody. The life span of plasma cells is few days and they secrete more amounts of antibodies during this time. These secreted molecules of antibodies are major effector for humoral immunity.
T lymphocytes also arise in bone marrow, but migrate to the thymus region for the maturation. These matured T cells exhibits a unique antigen binding molecule i.e., the T cell receptor on its membrane. The T cells are sub-divided into T helper cells (TH) and T cytotoxic cells (TC). These both can be differentiated from one another based on the presence of CD8 or CD4 membrane glycoproteins.
T cells that exhibit CD4 functions as T helper cells and those displays CD8 are T cytotoxic cells. Mostly the T cell receptors can recognize only antigen that is bound to cell membrane protein called major histocompatibility complex (MHC) molecules, there are two classes of MHC molecules; class I MHC molecules that are expressed in all nucleated cells and class II MHC molecules which are expressed by antigen presenting cells. After a TH cell recognizes and interacts with an antigen MHC class II molecules complex, the cell is activated and it undergoes metabolic transformation and begins to secrete various cytokines.
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