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. Immune system, Immunology Transplant rejection Tissue typing Major histocompatibility complex (MHC)
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."
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).
Multicellularity has arisen independently dozens of times in the history of life, in plants, animals, fungi, and prokaryotes, 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. 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.
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. 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.