A vile cough, soaring temperature? When attacked by nasty microbes, our immune system comes in handy. Surprisingly (or not), plants have come up with a very similar solution to dealing with pathogens, but independently...

 

 

All organisms face a constant onslaught from potential pathogens, and not surprisingly a variety of defence mechanisms have evolved. As ever, they show rampant convergences – the same challenge is met by much the same solution. Lines of defence can include antimicrobial compounds (e.g. peptides), but of great importance is the immune system (most obviously when it is compromised in such diseases as AIDS that allow secondary infections to take hold). In terms of immune systems, it is customary to distinguish between the innate and the adaptive system. Both are based on cell-mediated and antibody-mediated (humoral) components, but differ in a number of ways.

Innate immunity

The innate immune system, which is more primitive, is found in almost all organisms. It provides a non-specific and immediate response against pathogens (e.g. involving natural killer cells and macrophages in vertebrates), but lacks an “immunological memory” and does hence not bestow long-lasting protection. Although the widespread innate immune system may draw on some building blocks that are the same in all organisms, it clearly has emerged independently several times, most strikingly in plants and animals. In both these groups, we find the same basic architecture of receptors located on the margin of the cell (transmembrane receptors). But whilst animals employ TLRs (toll-like receptors), plants use different proteins (kinases) to achieve the same function. The receptors within the cell are not the same either, but have a strikingly similar tripartite arrangement including a shared domain at one end that is rich in the amino acid leucine (or LLR, leucine-rich repeats).

Adaptive immunity

While innate immunity provides the first line of defence and is generally highly effective, it does not have the flexibility of the highly specific adaptive immune system. Here, a microbial threat is not only recognised, but the system retains a “memory” of the threat, so it can be quickly dealt with if (and more likely when) it re-invades. This adaptive system is effectively clonal – once the threat (an antigen) is identified (which is rapidly achieved because the antibody genes can produce numerous alternative receptors to “test” the antigen), vast numbers of defensive antibodies are produced. The adaptive immune system is characteristic of vertebrates, but was long assumed to be limited to jawed vertebrates (gnathostomes). Only quite recently was it realised that the more primitive jawless fish (lampreys and hagfish) actually possess adaptive immunity that functions in much the same way as that found in the gnathostomes. However, it is based on a radically different system of antigen recognition. While the antigen receptors of gnathostomes are of the immunoglobulin type, jawless fish possess so-called variable lymphocyte receptors (VLRs) that are composed of proteins with leucine-rich repeats. Other convergences between the gnathostomes and jawless fish include the use of an enzyme (DNA cytidine deaminase) central in the antigenic response. So the adaptive immune system has only evolved twice, or has it? Albeit no match in its scope and flexibility, it appears that the immune system of insects is more versatile than previously thought. Not only does it have elements of a memory, but also a certain plasticity in response to microbial attack.

There are also deeper stories now emerging, some of which are vital not only for our general health but especially our mental well-being. It is becoming increasingly clear that in a number of ways there are important connections between our immune system and nervous system. Not only that, but the gut flora is unexpectedly important in early brain development. Once again, complexity not only emerges multiple times, but reiles on hitherto underappreciated symbioses between the microbes and their hosts.