Prevention is Better Than Cure
After the last few years, you might think we’re all experts on the immune system. Well maybe we are, but insect – particularly social insect – immune systems may surprise you. How do these highly social animals try to protect themselves from contagious diseases? We know it aint easy, so resident science writer Dave Black explains how bee immunity works, and what we can learn about our own immunity.
Honey bees maintain nice stable environments in their living quarters, and store plenty of food for long periods, so there are plenty of opportunities for pests and diseases to flourish. Closely related individuals live in close quarters, so pathogens could spread easily and rapidly given a chance. Beekeepers don’t often think in terms of individual immunity, our emphasis is on more visible, behavioural aspects of disease prevention like grooming, hygienic behaviour, removing corpses, absconding, propolis use, a collective ‘fever’ response, and self-removal by unhealthy individuals. We refer to these as aspects of ‘social immunity’.
However, individual bees do have immune systems that are not the same but, in many respects, not that dissimilar to ours. That is interesting because it suggests the basic mechanisms evolved a long time ago, and because they were so valuable have been conserved as species became more complex. The evidence lies in the common features of our genome that insects and vertebrates, honey bees and humans, share.
Insect immune systems are of course quite well studied, not least because humans have spent a lot of time trying to kill many of them. It's true that, compared to other insects, honey bee immunity is quite ‘primitive’, and that may be because it operates in a restricted (socially controlled) environment against a more limited range of threats. It appears that honey bees have not found it necessary to diversify their individual immune response the way other insects have had to .
Honey bees have several ways to defend themselves from pests and foreign chemicals. There are physical barriers that line both the inside and the outside of the animal, its exoskeleton and its antimicrobial cuticle, as well as mucus linings, friendly bacteria, and membranes in the gut.. They can also employ a cellular immune system deployed by cells known as haemocytes that circulate in the haemolymph (that’s the bee’s ‘blood’). There are several different kinds, and they can adhere to, break up, engulf, or encapsulate (‘melanise’) material and render it harmless.
Haemocytes use compounds produced by the salivary glands and the fat-body to make and store enzymes and peptides that interact with or damage the structure or chemistry of microbes and foreign elements. The suite of chemicals, referred to as antimicrobial peptides or AMPs, that circulate in the haemolymph used by the immune system, is known as the humoral response. ‘Humoral’ because it relates to the body’s ‘humors’, an old-fashioned name for the fluids a body contains. Honey bees produce far fewer AMPs than most other insects. In all, compared to other insects, honey bees are thought to have about a third of all the genes we know to be associated with innate immunity.
Despite the similarity, the immune system of insects differs from vertebrates like us in a very fundamental way. Insects have an ‘innate immunity’; today we might think of it as ‘hard-wired’. It’s inherited, co-evolved, specific and immutable (that means unchanging over time). By contrast, vertebrates have both innate immunity but also ‘adaptive’ immunity that allows us to recognise a broad range of pathogens, even novel ones, remember them, and produce a response specific to each threat. Vertebrate immune systems can learn, whereas insect immune systems evolve if they can. Or so we thought.
Perhaps we don’t know as much as we thought…
In 2003 a study published evidence of a small crustacean apparently being more resistant to a parasitic tapeworm in subsequent infections. Since then, what has become known as ‘immune priming’ has been offered as an explanation in cases involving mosquitos, fruit flies, a beetle, woodlouse, various worms , and, in 2006, Bombus terrestris – bumble bees. Together these studies suggest there are circumstances where innate immunity gets better at mounting a response, and ‘remembers’ to provide a specific, effective response after a period of time, at least weeks. We don’t yet know how this might work, and we don’t know if this can be transferred to the animal’s progeny, but there have been studies that indicate it might be [4, 5].
A different form of adaptive immunity may yet be shown to exist in insects, and for all we know, other organisms. That just means, (and this is a good thing) we don’t know as much about immune systems as we think we do.
Dave Black is a Bay of Plenty based hobbyist beekeeper who now works in the kiwifruit industry. He has a degree in Environmental Science and for the past 25 years he has been reading and writing about bees and beekeeping. His essays are available at www.beyondbeebooks.substack.com/
REFERENCES  J. D. Evans et al, Immune pathways and defence mechanisms in honey bees, Apis mellifera Insect Molecular Biology (2006) 15(5), 645–656  Joseph C Sun, Sophie Ugolini, & Eric Vivier, Immunological memory within the innate immune system. The EMBO Journal Vol 33 No 12 (2014). DOI 10.1002/embj.201387651  Ben M. Sadd, and Paul Schmid-Hempel, Insect Immunity Shows Specificity in Protection upon Secondary Pathogen Exposure. Current Biology 16, 1206–1210, June 20, (2006), DOI 10.1016/j.cub.2006.04.047  Hannah J. Tidbury, Amy B. Pedersen and Mike Boots, Within and transgenerational immune priming in an insect to a DNA virus. Proc. R. Soc. B (2011) 278, 871–876, doi:10.1098/rspb.2010.1517  Hernandez Lopez J, Schuehly W, Crailsheim K, Riessberger-Galle U. (2014) Trans-generational immune priming in honeybees. Proc. R. Soc. B 281: 20140454. doi.org/10.1098/rspb.2014.0454