Immunity: defense against infections essential for all living organisms

All organisms need protection against infection. Bacteria are often primarily seen as infectious agents, but they also need protection against bacterial viruses, so-called bacteriophages.

To this end, bacteria have developed very complex defense systems, including apoptosis-like mechanisms, restriction enzymes, and even adaptive-type mechanisms involving immunological memory of immune responses through a system called CRISPR–Cas. An earlier dominating view was that adaptive immunity in eukaryotes only exists in jawed vertebrates, as their immune system includes the classical and highly variable immunoglobulins (Igs) and T-cell receptors (TCR).

However, other types of variable molecules, which may be involved in immunity, have also been identified in insects, snails, lancelets, plants, sea urchins, and jawless fishes. Interestingly, fishes without jaws, such as the hagfish and lamprey, have a very complex adaptive immunity built on lymphocyte-like cells and variable lymphocyte receptors (VLRs).

Notably, the variability of these VLRs has been estimated to be in the same range as Igs and T-cell receptors. This illustrates that very diverse strategies have been used to create an adaptive immune system in different organisms, indicating potent convergent evolution.