Immune system is a remarkable defense mechanism to protect hosts from pathogens. It can be broadly divided into two arms, innate and adaptive. Although having some unique components, the two arms have extensive shared and overlapping mechanisms in common.
Innate immunity is fast in action utilizing pre-existing components to combat an infection; however, it is limited and has fixed specificity and diversity. Innate immunity cannot provide pathogen specific-long lasting immunity and there is no specific memory for the pathogen. Innate immunity includes various cell types including macrophages, dendritic cells, neutrophils, eosinophils, and natural killer cells (Kuby Immunology, Sixth Edition). The innate immune system responds immediately to infections, mechanical or chemical assaults which disturb tissue homeostasis. It recognizes a large variety of microbe- or pathogen-associated molecular patterns (MAMPs and PAMPs) as well as exogenous and endogenous non-microbial danger signals such as damage-associated molecular patterns (DAMPs). These danger signals trigger germline-encoded pattern recognition receptors (PRRs) such as the Toll-like receptors (TLRs). PRR triggering leads to an innate inflammatory response that aims at the restoration of homeostasis and repair of tissue damage. Moreover, this response is a prerequisite for efficient activation of the adaptive immune system, e.g., the priming of naive T cells by activated dendritic cells (DCs).
The other arm of the immune system, the adaptive immunity has a lag in its onset (ranging from seven days to two weeks depending on the organism and on the infectious agent) but compensates the delay with characteristic high specificity, enormous diversity, immunological memory and self-nonself recognition. Despite their differences, both arms of immune system work in a highly cooperative manner and such co-operativity is imperative in the generation of an effective immune response.
The adaptive immunity arm of the immune system is further divided into humoral immunity and cell-mediated immunity mediated predominantly by B cells and T cells respectively. The primary role of B cell mediated immunity is to detect and eliminate the extracellular pathogens. The principal role of T cell mediated immunity is to provide protection from intracellular pathogens and to eliminate damaged or non-typical cells (for example, tumor cells) (Kuby Immunology, Sixth Edition).
The adaptive immune system(AIS), also called the antibody based immune system, is characterized by the presence of molecules such as RAG, MHC, TCR and antibody, and by the use of gene rearrangement (VDJ-C recombination) to generate almost unlimited receptor diversity with only a limited number of genes. All the defining elements of the immune system, antibodies, T cell receptors (TCRs), MHC products and recombination activator genes (RAG), are present in sharks and rays, while no such a system can be found in Agnathan. Instead, agnathans, such as lamprey and hagfish, developed a completely different Leucine -Rich Repeat (LRR) immune receptor system, which utilizes a “gene-conversion (GC) like” mechanism to generate receptor diversity.
1. Martin S F. Adaptation in the innate immune system and heterologous innate immunity[J]. Cellular and Molecular Life Sciences, 2014, 71(21): 4115-4130.