Everyone eats vegetables but aside from the hottest chillis most of us don’t consider them all that dangerous. For a few people though, some fruits and vegetables can cause allergic reactions, all the way from itchy eyes and sneezing through to very dangerous anaphylactic shock. But why do our immune systems react this way to something that appears to be harmless?
From first appearances you wouldn’t think that celery and parasitic worms had much in common. However our immune systems see things differently.
During an infection by a virus, bacteria or parasite, the immune system produces antibodies, which help attract the right type of immune cell to the infecting organism to destroy it. Viruses and bacteria are small pathogens (compared with our immune cells) and so specialised antibodies (IgG or IgM) bind to specific proteins (antigens) on the surface of the pathogen. These attract the attention of immune cells such as neutrophils which then engulf and destroy them (see video).
| A crawling neutrophil chasing after a bacterium. Taken from a 16-mm movie made in the 1950s by the late David Rogers at Vanderbilt Universit |
However, parasites like blood flukes and worms are too big to be engulfed, and so a different class of antibody (IgE) attracts specific types of immune cells, such as mast cells and basophils, which release chemicals onto the parasite to attempt to destroy it. During an allergic reaction, the same IgE antibodies and immune cells are inappropriately activated by allergens. The chemicals they release – including histamine - cause damage to our own tissues and result in the itchy rashes and sneezing (or worse) we associate with allergy.
So why do the immune systems of people with allergies react to allergens in the way they do? There are lots of factors that result in a person having allergy, including their genetic make up and their environment. Our research is trying to find out what makes an allergen an allergen, or rather, why do we have allergy against some things but not others?
We are testing the hypothesis that the structures of known allergens can be used to predict which antigens in the parasite are the targets of the “allergy like” immune response. For example we have found that a celery allergen has a similar structure to a Schistosoma mansoni parasite molecule that we would predict to be targeted by an allergy like response in infected people. From the images of the molecules (below) we can see that they share a similar structure, with helical structures (red) either side of a flat sheet (green) . We believe that this similarity is key in deciding what makes both molecules targets of the allergy-like response. Similar relationships have also been found in other families of allergens including those from shellfish, house dust mites, insect venom, nuts and plants.
Interestingly, it has also been shown that some antigens on the surface of parasites are not the targets of this protective “allergy like” immune response. Understanding how the immune system sees these molecules and decides which ones to target with an “allergy like response” may help design new treatments to prevent people from developing allergy, or at the very least help convince your immune system that your tasty (?!) piece of celery isn’t a parasite.
Ed Farnell is a postdoctoral researcher in the Dunne Group in the Department of Pathology, and a CID Fellow.
Read more:
On this site: