From peanuts to pollen, some of the smallest and most unthreatening substances can sometimes be the deadliest, if you happen to be allergic to them. These seemingly overreactions are produced by the body’s immune system, which mistakes harmless chemicals for invading pathogens, and responds by initiating a number of extreme measures.

In most cases, this is coordinated by antibodies called Immonglobulin E (IgE), which are produced when the body detects the presence of a harmful intruder. IgE then stimulates cells throughout the body to release chemicals that bring about an immune response, which can manifest itself in a number of ways.

For example, pollen allergy – or hay fever – typically results in the airways becoming blocked as IgE causes cells to produce histamine. This, in turn, leads to an increase in the production of mucus, which then clogs up the nose and makes breathing difficult.

Interestingly, a recent study found that a protein called BetV1 – which happens to be the most common allergen in pollen – is extremely similar in structure to a protein produced by some parasitic worms, which could explain why the human body sometimes mistakes pollen for a pathogen.

Proteins found in peanuts, as well as chemicals used in medicines and even materials like latex, can stimulate allergic reactions as well. These reactions can take a number of forms, including skin rashes, nausea, and in extreme cases can result in anaphylactic shock.

Research has found that several factors contribute to a person’s likelihood of suffering from allergies. The hygiene hypothesis, for instance, states that people who are not exposed to allergens at a young age may be more susceptible to allergies when they get older. Though more evidence is needed to prove this theory, some say it explains why allergy rates have increased as cleanliness and sanitation have improved.

Scientists are also now advising people to expose their children to allergenic foods like peanuts and eggs at a young age, as studies have shown that babies who do not come into contact with these products are more likely to develop allergies when they eventually encounter them. Again, the evidence to support this claim is not rock-solid, but the idea of allowing the immune system to become familiar with these harmless substances as early as possible seems to make scientific sense.