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I have recently been doing a lot of research into the interplay between the innate and adaptive immune systems in humans, and mammalian laboratory models. This has led to my reading some interesting information on the immune response in insects;

Insects have a highly efficient immune system. In response to a bacterial attack, their fat body (the equivalent of the liver in mammals) synthesizes a whole range of peptides with an antibacterial and antifungal effect.

This fascinated me, as the clear inference is that there are no ‘dedicated’ immune cells, but that adipose tissue has far more diverse functions that I had realized.

I have done a little more reading, and also looked at plant immune systems, which seems far more analogous to those in insects than mammals;

Plants, unlike mammals, lack mobile defender cells and a somatic adaptive immune system. Instead, they rely on the innate immunity of each cell and on systemic signals emanating from infection sites. (Jones, 2006)

My questions relates to the need of an adaptive immune response in mammals. The immune systems in insects and plants - a more 'systemic' immunity due to the lack of dedicated/mobile immune cells - seems much simpler.

Given that evolution works incrementally (there are no 'jumps' - for instance, going from a non-dedicated immune system, to a dedicated immune system), I would hypothesise that organisms less distantly related to insects and plants may have tissues with duel functions (similar to insects?), but that specialize further as immune cells until gradually (down the evolutionary tree) a complex and specific immune system emerges. (This is complicated by the fact that our immune systems do have multiple roles - e.g. tissue remodelling, but I wasn't going to go into that here. Feel free in your answers if it is necessary!).

My overall curiosity can be summarized as 2 questions;

  1. What are the possible reasons why a dedicated and immensely complex immune system evolved in some lineages of organism?
  2. Is there any evidence of 'half-way' organisms, and about what ecological time-frame might the dedicated immune system have developed?
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Immunity has probably evolved more than once as it is completely necessary for survival. You might even argue that defenses such as antibiotics and predatory behavior are immune defenses for single celled creatures.

As you point out, plants have a different immune system than animals do. Insects do too. They don't have innate immune memory which many animals have, where pools of cells embody the immune response to a specific antigen. (i.e. encode for a specific immunoglobin or a tCell Receptor variant).

I found this reference that this appears to be the collective property of jawed vertebrates. Even so, there are many late innovations in the immune system, where reptilian and mammalian immune responses vary substantially.

Its not clear to me whether other animals have the insect system. Animals that branched off before insect lines such as radiates probably have their own way of dealing with immune response. That's just a guess though.

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Immunology is a bio-chemical and nebulous concept, however it's equally complicated as the nervous, structural and circulatory morphology and chemistry. You can compare the immune system complexity to visible complexity that evolved in animals, from simple spiral and pattern shells, to metameric (repetitive segment) shells with a small amount of articulations like crabs and experimental Archean animals, to highly articulated shrimp and scorpions, and then to thousands of complex scales with specialized points and stings, which even become an endoskeleton. The evolution of the first muscled and articulated endoskeletons is as confusing as the immune system but it also happened gradually, before there were bony fish there were cartilaginous ones like sharks. In the same way, the immune system has evolved in strange steps from simple to highly complex and made of many pieces.

Bacteria have kinds of immune systems to protect against viruses, which consist simply of DNA tricks to randomly re-arrange it's DNA and so neutralize the dangerous virus DNA which is copied within them and inherit new genes without making virii... If you want to go a step up from that in evolution complexity, then study immunology of worms and immunity of sponges.

The battle between bacteria and bacteria-eating viruses has been going on for millions of years, with viruses attempting to replicate themselves by -- in one approach -- invading bacteria cells and integrating themselves into the chromosomes of the bacteria. When this happens a bacterium makes a copy of its chromosome, which includes the virus particle. The virus then can choose at a later time to replicate itself, killing the bacterium -- similar to a ticking time bomb, Wood says.

However, things can go radically wrong for the virus because of random but abundant mutations that occur within the chromosome of the bacterium. Having already integrated itself into the bacterium's chromosome, the virus is subject to mutation as well, and some of these mutations, Wood explains, render the virus unable to replicate and kill the bacterium.

With this new diverse blend of genetic material, Wood says, a bacterium not only overcomes the virus' lethal intentions but also flourishes at a greater rate than similar bacteria that have not incorporated viral DNA.

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As you guessed, the immune system as we think of it in humans has evolved incrementally over hundreds of millions of years. The components that most people think of as "immune system" (T cells, B cells, antibodies) arose in sharks, around 400 million years ago. Lampreys and hagfish, which split off from the rest of the vertebrate lineage before sharks developed their adaptive immune system, have parallel solutions to the problem that are mostly independent but that involve some evolutionarily related structures.

The "innate" immune system is much more ancient. Humans and other vertebrates share parts of the innate immune system with insects and more evolutionarily diverse organisms, so these arose well over 500 million years ago.

The topic is reasonably well understood and far too broad to discuss in any detail here. I've written a couple of summaries elsewhere:

Here are some review articles:

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