Anecdotal evidence abounds that being very sick, from either a viral or a bacterial infection, can change the way an individual experiences allergies. In some cases, people even claim to have developed a brand new allergy after being ill. Is there any truth to this? The science says yes, at least some.
Infections and Allergies Have the Immune System in Common
Both allergic reactions to otherwise harmless substances and infections caused by a virus or bacteria involve action of the immune system, which is the first clue that one may impact the other. When the body’s immune system detects a foreign invader, it goes into attack mode to clear it out. Typically this is a harmful type of bacteria or a virus.
In the case of allergies, the immune system mistakes something harmless, like a food protein or pollen, for something dangerous. It goes on the attack, and this causes the characteristic symptoms of an allergic reaction.
The Hygiene Hypothesis
One of the most prevalent hypotheses for why allergies are on the rise, and why they seem to affect developed nations and populations the most, is related to illness and infection. The rise in allergies has seemed to coincide with an increase in cleanliness, use of antibiotics and use of antibacterial products, like hand sanitizer and soap, which has led to the idea that when kids are not exposed to infectious agents, they may be more prone to developing allergies.
A Parasite May Change Allergic Reactions
Parasites are microscopic organisms that can cause infections, and there is evidence from research that a particular type of parasitic worm may actually reduce the severity of allergic reactions. Helminths include tapeworms, nematodes, roundworms and others. Infection by a helminth induces a particular type of immune response, but multiple studies have found that it can also protect against allergies.
For example, in one study mice with an immune response to peanut protein were infected with helminths, while others were left uninfected. The mice were then exposed to peanut extract. Those without the infection had typical responses to the allergen. Those that were infected with the parasite either had no response to the peanut or had an allergic reaction but with reduced symptoms.
Why this happens is not fully understood, but researchers speculate that the parasite blocks the antibodies the immune system normally produces against the allergen in order to protect its host and therefore to protect itself. The findings from this and other studies may support the hygiene hypothesis of allergies.
The Common Cold and Increased Allergic Reaction
Other research has found an opposite effect with the virus that causes the common cold. In one study, patients with known allergies were infected with the rhino virus and exposed to their allergens before infection, during the acute illness and after recovering from the illness. All of the patients experienced greater release of histamine, the immune system factor that produces many allergic symptoms, as well as greater production of immune system white blood cells.
The patients used as control subjects, those without allergies or those with allergies who were exposed to a placebo instead of their allergens, did not have the same enhanced allergic response. This study indicates that an acute upper respiratory infection can worsen an allergic reaction in susceptible individuals.
Viral Infections and Drug Allergies
There is also evidence that viral infections can trigger or worsen allergic reactions to medications. For example, having HIV or being infected with the Epstein-Barr virus is a known risk factor for being allergic to medications. This may be a result of repeated exposure to certain drugs used to treat the infection. Studies have found that an allergic reaction to drugs, known as a skin rash, or drug-induced hypersensitivity syndrome, is also related to viral infections, specifically those caused by herpes viruses like Epstein-Barr, or human herpesvirus-6.
The science of infection, illness and allergies is still being discovered, but it is clear that there is a connection. And, it seems that the effects may go both ways: some infections may protect against allergic reactions, while others worsen them.