Allorecognition is the ability of an individual organism to distinguish its own tissues from those of another. It manifests itself in the recognition of antigens expressed on the surface of cells of non-self origin. Allorecognition has been described in nearly all multicellular phyla.
This article focuses on allorecognition from the standpoint of its significance in the evolution of multicellular organisms. For other articles which focus on its importance in medicine, molecular biology, and so forth, the following topics are recommended as well as those in the Categories links at the bottom of this page.
The ability to discriminate between self and non-self is a fundamental requirement for life. At the most basic level, even single-celled organisms need to be able to distinguish between food and non-food, to respond appropriately to invading pathogens, and to avoid cannibalism. In sexually reproducing organisms, self/non-self discrimination is essential to ensuring species-specific egg/sperm interaction during fertilization. Hermaphroditic organisms, such as annelids and certain plants, require recognition mechanisms to prevent self-fertilization. Such functions are all carried out by the innate immune system, which employs evolutionarily conserved pattern recognition receptors to eliminate cells displaying "nonself markers."[1]
Evolution of multicellularity
The evolution of multicellularity brought about various challenges, many of which could be met by increasingly sophisticated innate immune systems, but which also served as an evolutionary driving force for the development of adaptive immune systems. The adaptive or "specific" immune system in its fully qualified form (i.e. based on major histocompatibility complex (MHC), T-cell receptors (TCR), and antibodies) exists only in jawed vertebrates, but an independently evolved adaptive immune system has been identified in hagfish and lampreys (non-jawed vertebrates).[2]
Multicellularity has arisen independently dozens of times in the history of life, in plants, animals, fungi, and prokaryotes,[3] appearing first several billion years ago in cyanobacteria. Two categories of advantages have been attributed to the early development of multicellular existence: advantages related to size, and advantages related to functional specialization and division of labor.[4] Size advantages may include greater feeding efficiency or increased robustness. For example, myxobacteria, moving in swarms, are able to maintain a high concentration of extracellular enzymes used to digest food, from which all the bacteria in the swarm benefit. Under various conditions, many microorganisms form biofilms which provide them with a protected environment. In organisms that have evolved functional specialization, an important division of labor may exist over reproduction: only a small fraction of cells contribute to the next generation. Somatic growth represents a form of altruism, where somatic cells give up reproduction helping germline cells reproduce.[citation needed]
Free rider problem
The extracellular enzymes secreted by swarming bacteria, the slime of a biofilm, or the soma cells in a differentiated organism represent public goods which are vulnerable to exploitation by cheaters.[5] This issue is well known in economics and evolutionary biology as the "free rider problem" or the "tragedy of the commons." A free rider (or freeloader) is an individual that consumes a resource without paying for it, or pays less than the full cost. In multicellular organisms, cheaters may arise from mutations in somatic cells that no longer contribute to the common good, or ignore controls on their reproduction. Another possibility may arise from somatic fusion: there are multicellular life-styles where there are few if any physical barriers to the intermingling of cells (for example: sponges, fungal mycelia) and even among organisms that have evolved physical integuments representing a first line of defense against invasion, opportunities for cellular exchange occur. Witness, for example, the spread of devil facial tumour disease among Tasmanian devils and transmissible venereal tumor in dogs.[citation needed]
In metazoans, defense against disruption of the multicellular life style by such cheaters takes two major forms. First, a consistent feature of the multicellular life cycle is the interposition of a unicellular phase, even among organisms whose major mode of propagation may be via many-celled vegetative propagules.[6] This unicellular phase usually takes the form of a sexually produced zygote. Passage through a unicellular bottleneck assures that each representative of the next generation of organisms represents a distinct clone. Some offspring will carry a large number of deleterious mutations and will die off, while other offspring will carry few. In this manner, the organism bypasses "Muller's ratchet," the process by which the genomes of an asexual population accumulate deleterious mutations in an irreversible manner. The second defense against cheaters is the development of allorecognition mechanisms that guard against invasion by parasitic replicators.[7] Allorecognition acts as an agent of kin selection by restricting fusion and community acceptance to related individuals. If related individuals fuse, the benefits of fusion will still apply, while the costs of competition for shared resources or reproductive opportunities will be reduced by a fraction proportional to the degree of relatedness between the fusing partners.[5] If unrelated individuals fuse, or if a mutated cell arises within an organism that is distinguishable from self by the allorecognition system, a rejection response will be activated. As a general rule, rejection is mediated by the gene products of highly variable loci, which must match (or nearly match) between organisms for fusion to be successful.[8]
Allorecognition phenomena
Allorecognition phenomena have been recognized in bacterial self-identity and social recognition systems,[9] kin discrimination in social amoebae,[10][11] fungal mating types,[12] fungal vegetative incompatibility,[13] plant self-incompatibility systems,[14] colonial marine invertebrates (such as corals, sponges, hydroids, bryozoans, and ascidians),[15] and of course, vertebrates. The manner in which allorecognition manifests itself in these different systems varies greatly. Bacteria, for instance, secrete bacteriocins, proteinaceous toxins specifically targeted against members of their own species. Colonies of marine invertebrates, each representing a single genotype, expand across the ocean floor by asexual reproduction. Where colonies meet, they may, if compatible, fuse to form a single unit, or if incompatible, they may aggressively attempt to overgrow, poison, sting, or consume each other.[15]
Innate and adaptive immune systems
Vertebrate immunity is dependent on both adaptive and innate immune systems. In vertebrates, the innate immune system is composed of cells such as neutrophils and macrophages (which also have a role in the adaptive immune system as antigen presenting cells), as well as molecular pathways such as the complement system which react to microbial non-self. The innate immune system enables a rapid inflammatory response that contains the infection, and it activates the adaptive immune system, which eliminates the pathogen and, through immunological memory, provides long term protection against reinfection.[citation needed]
Comprehensive sequence searches across multiple taxonomic groups have failed to identify MHC and TCRs outside of the jawed vertebrates. Allorecognition in these animals rely on molecular mechanisms distinct from those of the jawed vertebrates. In sponges, various receptors (sponge adhesion molecules, receptor tyrosine kinase) with domains similar to those found in immunoglobulins have been identified. Sequence variability in "hot spots" have been identified in these receptors.[2] It would appear that molecules which, later in evolution, were exploited in the adaptive immune response, had an earlier role in innate recognition. Lampreys and hagfish appear to have evolved, by convergent evolution, an adaptive immune response that is independent and distinct from the adaptive immune systems of higher vertebrates. Lymphocyte-like cells in these fish express highly variable lymphocyte receptor genes, which undergo somatic rearrangements reminiscent of the manner in which mammalian immunoglobulin genes are rearranged during development.[16]
Summary
In summary, allorecognition, the ability to distinguish self from non-self, is basic to all life, unicellular as well as multicellular. The earliest recognition systems were innate, and were based on the recognition of self molecules. The evolution of multicellular forms brought about selective pressures for ever-increasing sophistication to innate immune systems. Adaptive immune systems, based on the recognition of non-self, have arisen independently in two lines of chordates, and exploit molecules and cellular systems which had a previous role in innate immune responses. Allorecognition as it currently exists in mammals can be traced back as the result of sequential modification to immunity mechanisms dating back to some of the earliest multicellular organisms.
↑ Plants have a third, physical line of defense. In animals, cancers can spread through the circulatory system to other parts of the organism. Plants do not have such circulatory systems and their cells are fixed in place. Hence, tumors in plants (known as galls) are generally limited to a small part of the plant.
The cell is the basic structural and functional unit of life. Every cell consists of a cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids.
The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splinters, distinguishing them from the organism's own healthy tissue. Many species have two major subsystems of the immune system. The innate immune system provides a preconfigured response to broad groups of situations and stimuli. The adaptive immune system provides a tailored response to each stimulus by learning to recognize molecules it has previously encountered. Both use molecules and cells to perform their functions.
A somatic cell, or vegetal cell, is any biological cell forming the body of a multicellular organism other than a gamete, germ cell, gametocyte or undifferentiated stem cell.
In biology, immunity is the capability of multicellular organisms to resist harmful microorganisms. Immunity involves both specific and nonspecific components. The nonspecific components act as barriers or eliminators of a wide range of pathogens irrespective of their antigenic make-up. Other components of the immune system adapt themselves to each new disease encountered and can generate pathogen-specific immunity.
A multicellular organism is an organism that consists of more than one cell, in contrast to a unicellular organism.
Toll-like receptors (TLRs) are a class of proteins that play a key role in the innate immune system. They are single-pass membrane-spanning receptors usually expressed on sentinel cells such as macrophages and dendritic cells, that recognize structurally conserved molecules derived from microbes. Once these microbes have breached physical barriers such as the skin or intestinal tract mucosa, they are recognized by TLRs, which activate immune cell responses. The TLRs include TLR1, TLR2, TLR3, TLR4, TLR5, TLR6, TLR7, TLR8, TLR9, TLR10, TLR11, TLR12, and TLR13. Humans lack genes for TLR11, TLR12 and TLR13 and mice lack a functional gene for TLR10. TLR1, TLR2, TLR4, TLR5, TLR6, and TLR10 are located on the cell membrane, whereas TLR3, TLR7, TLR8, and TLR9 are located in intracellular vesicles.
A heterokaryon is a multinucleate cell that contains genetically different nuclei. Heterokaryotic and heterokaryosis are derived terms. This is a special type of syncytium. This can occur naturally, such as in the mycelium of fungi during sexual reproduction, or artificially as formed by the experimental fusion of two genetically different cells, as e.g., in hybridoma technology.
The adaptive immune system, also known as the acquired immune system, is a subsystem of the immune system that is composed of specialized, systemic cells and processes that eliminate pathogens or prevent their growth. The acquired immune system is one of the two main immunity strategies found in vertebrates.
Pathogen-associated molecular patterns (PAMPs) are small molecular motifs conserved within a class of microbes. They are recognized by toll-like receptors (TLRs) and other pattern recognition receptors (PRRs) in both plants and animals. A vast array of different types of molecules can serve as PAMPs, including glycans and glycoconjugates.
Pattern recognition receptors (PRRs) play a crucial role in the proper function of the innate immune system. PRRs are germline-encoded host sensors, which detect molecules typical for the pathogens. They are proteins expressed, mainly, by cells of the innate immune system, such as dendritic cells, macrophages, monocytes, neutrophils and epithelial cells, to identify two classes of molecules: pathogen-associated molecular patterns (PAMPs), which are associated with microbial pathogens, and damage-associated molecular patterns (DAMPs), which are associated with components of host's cells that are released during cell damage or death. They are also called primitive pattern recognition receptors because they evolved before other parts of the immune system, particularly before adaptive immunity. PRRs also mediate the initiation of antigen-specific adaptive immune response and release of inflammatory cytokines.
In cell biology, a phagosome is a vesicle formed around a particle engulfed by a phagocyte via phagocytosis. Professional phagocytes include macrophages, neutrophils, and dendritic cells (DCs).
Immunopathology is a branch of medicine that deals with immune responses associated with disease. It includes the study of the pathology of an organism, organ system, or disease with respect to the immune system, immunity, and immune responses. In biology, it refers to damage caused to an organism by its own immune response, as a result of an infection. It could be due to mismatch between pathogen and host species, and often occurs when an animal pathogen infects a human.
The innate, or nonspecific, immune system is one of the two main immunity strategies in vertebrates. The innate immune system is an older evolutionary defense strategy, relatively speaking, and is the dominant immune system response found in plants, fungi, insects, and primitive multicellular organisms.
Pathogenic fungi are fungi that cause disease in humans or other organisms. Approximately 300 fungi are known to be pathogenic to humans. Markedly more fungi are known to be pathogenic to plant life than those of the animal kingdom. The study of fungi pathogenic to humans is called "medical mycology". Although fungi are eukaryotic, many pathogenic fungi are microorganisms. The study of fungi and other organisms pathogenic to plants is called plant pathology.
Cell–cell recognition is a cell's ability to distinguish one type of neighboring cell from another. This phenomenon occurs when complementary molecules on opposing cell surfaces meet. A receptor on one cell surface binds to its specific ligand on a nearby cell, initiating a cascade of events which regulate cell behaviors ranging from simple adhesion to complex cellular differentiation. Like other cellular functions, cell-cell recognition is impacted by detrimental mutations in the genes and proteins involved and is subject to error. The biological events that unfold due to cell-cell recognition are important for animal development, microbiomes, and human medicine.
Holomycota or Nucletmycea are a basal Opisthokont clade as sister of the Holozoa. It consists of the Cristidiscoidea and the kingdom Fungi. The position of nucleariids, unicellular free-living phagotrophic amoebae, as the earliest lineage of Holomycota suggests that animals and fungi independently acquired complex multicellularity from a common unicellular ancestor and that the osmotrophic lifestyle was originated later in the divergence of this eukaryotic lineage. Opisthosporidians is a recently proposed taxonomic group that includes aphelids, Microsporidia and Cryptomycota, three groups of endoparasites.
The danger model is a theory of how the immune system works. It is based on the idea that the immune system does not distinguish between self and non-self, but rather between things that might cause damage and things that will not.
Jawless vertebrates, which today consist entirely of lampreys and hagfish, have an adaptive immune system similar to that found in jawed vertebrates. The cells of the agnathan AIS have roles roughly equivalent to those of B-cells and T-cells, with three lymphocyte lineages identified so far:
Effector-triggered immunity (ETI) is a property of the innate immune system. ETI was first identified in plants but has also been identified in animal cells.
A somatic mutation is a change in the DNA sequence of a somatic cell of a multicellular organism with dedicated reproductive cells; that is, any mutation that occurs in a cell other than a gamete, germ cell, or gametocyte. Unlike germline mutations, which can be passed on to the descendants of an organism, somatic mutations are not usually transmitted to descendants. This distinction is blurred in plants, which lack a dedicated germline, and in those animals that can reproduce asexually through mechanisms such as budding, as in members of the cnidarian genus Hydra.
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