Fish Immunology (1+1)

Unit 2 - Evolution of immune system

2.1. Evolution of immune system
Innate immune mechanisms can be found in species at almost every level of the evolutionary tree of life.
When life evolved the primitive organisms like of bacteria, archaea and eukaryotes brought change in environment, also increased the concentration of atmospheric oxygen, this facilitated the evolution of multicellular organisms (metazoans) around 600 million years ago. Evolution in multicellular organisms provided new host opportunities for microbial pathogens so these multicellular organisms developed new mechanisms of defence to protect themselves form pathogens, thus the origin of defence mechanisms begin and evolved along with the life.
The most primitive metazoans like sponges and coelenterates possess an epithelial layer of cells within an intermediate mesogleal layer which performs the role of digestive cells and defensive cells that engulf foreign organisms. These phagocytic cells lead the way for the development of vertebrate macrophage. This innate immunity uses germline encoded pattern recognition receptors for pathogens to distinguish between self and foreign.
Second layer of complex immune defences called adaptive immunity evolved in vertebrates around 500 million years ago. The adaptive immunity have not evolved overnight but it took several years. The unique feature of an adaptive immune system is the development of lymphocytes, where each lymphocyte possess an antigen recognition receptor that can be used to trigger specific defence mechanism. This lymphocytes are the key cells in evolution of specific immune system. Lymphoid cells found first in pre‑vertebrate named Deuterostomes. Later these lymphocytes evolved along with the life and the development of the diverse lymphocyte receptor allows vertebrates to recognize almost any potential pathogen or toxin and can generate antigen-specific responses to it. Antigen-activated lymphocytes differentiate into mature lymphocytes with cytotoxic and pro-inflammatory functions or into plasma cells that secrete antibodies and also provide protective memory of the antigen to fight the pathogen in future invasion.
Two types of adaptive immune system have evolved in vertebrates:

• Adaptive immune system in jawless vertebrates (hagfish and lamprey)
• Adaptive immune system of jawed vertebrates

Adaptive immune system in jawless vertebrates (hagfish and lamprey)
Lymphocytes present in jawless fish are indistinguishable from mammalian cells. But these Fish lymphocytes lack MHC (major histocompatibility complex) molecules, T-cell receptors and B-cell receptors. So jawless fishes use different types of antigen recognition receptor but use similar lymphocyte differentiation process to elicit specific immune response.
Adaptive immune system of jawed vertebrates
MHC (major histocompatibility complex) molecules, T-cell receptors and B-cell receptors based adaptive immune system is detected in cartilaginous fish but absent in lower chordates. For the evolution of adaptive immune system in jawed vertebrates it is believed that two macroevolutionary events that had provided the plat form for evolution of specific immune system are:-

• The invasion of recombination-activating gene transposon (RAG transposon)
• Whole-genome duplication (WGDs)