Many of the classical molecules of the adaptive immune system (for example, immunoglobulins and T-cell receptors) exist only in jawed vertebrates. A distinct lymphocyte-derived molecule has been discovered in primitive jawless vertebrates, such as the lamprey and hagfish. These animals possess a large array of molecules called Variable lymphocyte receptors (VLRs) that, like the antigen receptors of jawed vertebrates, are produced from only a small number (one or two) of genes. These molecules are believed to bind pathogenic antigens in a similar way to antibodies, and with the same degree of specificity. Manipulation by pathogens
The success of any pathogen depends on its ability to elude host immune responses. Therefore, pathogens evolved several methods that allow them to successfully infect a host, while evading detection or destruction by the immune system. Bacteria often overcome physical barriers by secreting enzymes that digest the barrier, for example, by using a type II secretion system. Alternatively, using a type III secretion system, they may insert a hollow tube into the host cell, providing a direct route for proteins to move from the pathogen to the host. These proteins are often used to shut down host defenses.An evasion strategy used by several pathogens to avoid the innate immune system is to hide within the cells of their host (also called intracellular pathogenesis). Here, a pathogen spends most of its life-cycle inside host cells, where it is shielded from direct contact with immune cells, antibodies and complement.
The classification system (Types A, B, and C) were defined by Celoria and Patton in 1959.The first successful repair of a Type A interrupted aortic arch was reported in 1961, in which the left subclavian artery was grafted into the descending thoracic aorta in a 14-year-old male patient. The first successful repair of a Type A interrupted aortic arch in an infant was in a 12-day-old infant in 1969, in which the left subclavian artery was connected to the descending thoracic aorta, the patent ductus arteriosus was closed, and the main pulmonary artery was banded.The first successful repair of a Type B interrupted aortic arch was in 1954, in which the 16 year-old, female patients own aorta was grafted from the arch to the descending thoracic aorta and the left subclavian artery was ligated. The first successful repair on a type B interrupted aortic arch in an infant was in 1973, in which a vein was used to connect the ascending aorta and the descending aorta.The first successful repair of a Type C interrupted aortic arch was in 1964, in which the 16-year-old female patients ascending aorta was grafted to the descending thoracic aorta. As of 1984, there had been no successful repairs on infants under one year old.The use of PGE1 dramatically improved the mortality rate after its introduction in 1976.