Reptilian Immune System

Rashika El Ridi, Zoology Department, Faculty of Science, Cairo University, Cairo, Egypt

Reptiles evolved from primitive amphibians nearly 300 million years ago during the Carboniferous period. They sprang from a basic stock, the long-extinct order Cotylosauria or 'stem reptiles'. A side-branch of stem reptiles led to the turtles and tortoises: order Testudines = Chelonia. The closest surviving descendants of the cotylosaurs are the tuatara that live on a few islands off New Zealand. Descended from ancient forms related to the tuatara is the much more successful order of the Squamata comprising the lizards (suborder Lacertilia) and snakes (suborder Ophidia = Serpentes). Stem reptiles also gave rise to other lines. One of these lines led to the crocodilians and modern birds; another led to the mammals. Reptiles are not only the closest ancestors of the birds and mammals, but are also the first vertebrates to exhibit the change from free-living larvae, as in fish and amphibians, to an embryo completely protected by an amnion similar to that found in avian and mammalian embryos. Therefore, study of immune processes in modern-day reptiles holds the promise of offering significant clues to the origin of certain facets of the immune response.

Reptiles are moderately abundant in the tropics, but less common in temperate zones and absent from cold climates. They are often said to be 'coldblooded' like fish and amphibians as opposed to the 'warm-blooded' birds and mammals. This means that reptiles lack the ability to regulate their metabolic heat for the production of sustained body warmth and a constant temperature. So, their temperature changes with that of their surroundings, hence the name poikilotherms, and they depend on an external source, rather than their own metabolism, for body warmth, i.e. they are ectothermic. Consequently, the structure and functional activity of several reptilian organs, including those of the immune system, are greatly modulated by environmental conditions.

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