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.
Such functions are all carried out by the innate immune system, which employs evolutionarily conserved pattern recognition receptors to eliminate cells displaying "nonself markers.
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.
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.
[5] 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.
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.
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] 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.
[11] Allorecognition phenomena have been recognized in bacterial self-identity and social recognition systems,[12] kin discrimination in social amoebae,[13][14] fungal mating types,[15] fungal vegetative incompatibility,[16] plant self-incompatibility systems,[17] colonial marine invertebrates (such as corals, sponges, hydroids, bryozoans, and ascidians),[18] and of course, vertebrates.
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.