From the humble amoeba searching for food (top left) to the mammal with its sophisticated humoral and cellular immune mechanisms (bottom right), all cellular organisms can discriminate between self and non-self, and have developed defence systems to prevent their cells and tissues being colonized by parasites.
This figure shows some of the important landmarks in the evolution of immunity. As most advances, once achieved, persist in subsequent species, they have for clarity been shown only where they are first thought to have appeared. It must be remembered that our knowledge of primitive animals is based largely on study of their modern descendants, all of whom evidently have immune systems adequate to their circumstances.
All multicellular organisms, including plants, have evolved a variety of recognition systems that respond to common molecular patterns found on the surface of microbes (e.g. lipopolysaccharides) by stimulating a variety of antimicrobial responses. This broadly corresponds to vertebrate innate immunity. In contrast, only vertebrates appear to have evolved adaptive immunity (characterized by specificity and memory), mediated by lymphocytes and three separate recognition systems (see Fig. 3): molecules expressed on B cells only (antibody), on T cells only (the T-cell receptor) and on a range of cells (the MHC), all of which look as if their genes evolved from a single primitive precursor (for further details see Fig. 10). Why only vertebrates have evolved adaptive immunity has never been totally explained, but there is a growing appreciation that the adaptive immune system brings with it very significant evolutionary costs. These include energy demands in maintaining the system (the human immune system has at least as many cells as the human nervous system), and also the potential danger that excess immunity will lead to tissue damage (as outlined in Figs 34–39). One of the consequences of the evolutionary quest to balance the pros and cons of the immune system is reflected in the extraordinary evolutionary diversity and genetic variability in many families of molecules involved in immune function (see Fig. 47).
Unicellular Organisms
Bacteria
We think of bacteria as parasites, but they themselves can be infected by specialized viruses called bacteriophages and have developed sophisticated systems to prevent this. It is thought that the restriction endonucleases, so indispensable to the modern genetic engineer, have as their real function the recognition and destruction of viral DNA without damage to that of the host bacterium. Successful bacteriophages have evolved resistance to this, a beautiful example of innate immunity and its limitations.
Protozoa
Lacking chlorophyll, these little animals must eat. Little is known about how they recognize ‘food’, but their surface proteins are under quite complex genetic control.
Invertebrates
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