Abstract

All animals, vertebrate or invertebrates, faces the challenges of combating pathogens while maintaining a tolerant relationship with symbiotic microorganisms. Tolerance to symbiotic is not a static or inert interaction but rather, required continuous active regulation of the front lines for these interactions with the mucosal surfaces. A fundamental question is whether or not specialized immune cells or organs have evolved in all animals to cope with the unique problems of mucosal defence, or whether specialized mucosal immunity is unique to vertebrates such as mammals [1]. This question can be answered by investigating the phylogeny of the mucosal and systemic immune system, from invertebrates to fish, amphibians, reptiles and birds to prototherians, the metatherians and eutherians. There is also, an extremely practical reason for studying phylogeny of many species and suffer from diseases of mucosal surfaces so that producing mucosal vaccines against fish pathogens is arguably as important to humankind as producing vaccines against the mucosal pathogens of mammals. Most of the paradigms for immune defences and symbiosis were established from studies of humans and a few other mammalian species such as rodents. Although these studies have provided more than a century of discovery and progress, much has also been learned from non-mammalian vertebrates as well as invertebrates, particularly over the past two decades [2-4].

The unique aspect of adaptive immune system is the generation of diverse receptors from germ line DNA. Among the animal phyla, the chordates arguably exhibit the greatest diversity in immune systems. Vertebrates are divided into Gnathostomes and Agnathans. These two lineages have adaptive immune systems that followed very different evolutionary paths while maintaining some common features. Both lineages use lymphocytes as the major cell type mediating adaptive immunity [5].