One way to prevent allergic rhinitis is to wear a respirator or mask when near potential allergens.

Growing up on a farm and having multiple brothers and or sisters decreases the risk.

Allergic diseases are strongly familial: identical twins are likely to have the same allergic diseases about 70% of the time; the same allergy occurs about 40% of the time in non-identical twins. Allergic parents are more likely to have allergic children, and those children's allergies are likely to be more severe than those in children of non-allergic parents. Some allergies, however, are not consistent along genealogies; parents who are allergic to peanuts may have children who are allergic to ragweed. It seems that the likelihood of developing allergies is inherited and related to an irregularity in the immune system, but the specific allergen is not.

The risk of allergic sensitization and the development of allergies varies with age, with young children most at risk. Several studies have shown that IgE levels are highest in childhood and fall rapidly between the ages of 10 and 30 years. The peak prevalence of hay fever is highest in children and young adults and the incidence of asthma is highest in children under 10.

Overall, boys have a higher risk of developing allergies than girls, although for some diseases, namely asthma in young adults, females are more likely to be affected. These differences between the sexes tend to decrease in adulthood.

Ethnicity may play a role in some allergies; however, racial factors have been difficult to separate from environmental influences and changes due to migration. It has been suggested that different genetic loci are responsible for asthma, to be specific, in people of European, Hispanic, Asian, and African origins.

Risk factors for allergy can be placed in two general categories, namely host and environmental factors. Host factors include heredity, sex, race, and age, with heredity being by far the most significant. However, there have been recent increases in the incidence of allergic disorders that cannot be explained by genetic factors alone. Four major environmental candidates are alterations in exposure to infectious diseases during early childhood, environmental pollution, allergen levels, and dietary changes.

Allergic rhinitis is the type of allergy that affects the greatest number of people. In Western countries, between 10 and 30 percent of people are affected in a given year. It is most common between the ages of twenty and forty.

Nonallergic rhinitis refers to rhinitis that is not due to an allergy. The category was formerly referred to as vasomotor rhinitis, as the first cause discovered was vasodilation due to an overactive parasympathetic nerve response. As additional causes were identified, additional types of nonallergic rhinitis were recognized. Vasomotor rhinitis is now included among these under the more general classification of nonallergic rhinitis. The diagnosis is made upon excluding allergic causes. It is an umbrella term of rhinitis of multiple causes, such as occupational (chemical), smoking, gustatory, hormonal, senile (rhinitis of the elderly), atrophic, medication-induced (including rhinitis medicamentosa), local allergic rhinitis, non-allergic rhinitis with eosinophilia syndrome (NARES) and idiopathic (vasomotor or non-allergic, non-infectious perennial allergic rhinitis (NANIPER), or non-infectious non-allergic rhinitis (NINAR).

In vasomotor rhinitis, certain nonspecific stimuli, including changes in environment (temperature, humidity, barometric pressure, or weather), airborne irritants (odors, fumes), dietary factors (spicy food, alcohol), sexual arousal, exercise, and emotional factors trigger rhinitis. There is still much to be learned about this, but it is thought that these non-allergic triggers cause dilation of the blood vessels in the lining of the nose, which results in swelling and drainage.

Non-allergic rhinitis can co-exist with allergic rhinitis, and is referred to as "mixed rhinitis." The pathology of vasomotor rhinitis appears to involve neurogenic inflammation and is as yet not very well understood. Vasomotor rhinitis appears to be significantly more common in women than men, leading some researchers to believe that hormones play a role. In general, age of onset occurs after 20 years of age, in contrast to allergic rhinitis which can be developed at any age. Individuals with vasomotor rhinitis typically experience symptoms year-round, though symptoms may be exacerbated in the spring and autumn when rapid weather changes are more common. An estimated 17 million United States citizens have vasomotor rhinitis.

Drinking alcohol may cause rhinitis as well as worsen asthma (see alcohol-induced respiratory reactions). In certain populations, particularly those of East Asian countries such as Japan, these reactions have a nonallergic basis. In other populations, particularly those of European descent, a genetic variant in the gene that metabolizes ethanol to acetaldehyde, ADH1B, is associated with alcohol-induced rhinitis. It is suggested that this variant metabolizes ethanol to acetaldehyde too quickly for further processing by ALDH2 and thereby leads to the accumulation of acetaldehyde and rhinitis symptoms. In these cases, alcohol-induced rhinitis may be of the "mixed rhinitis" type and, it seems likely, most cases of alcohol-induced rhinitis in non-Asian populations reflect true allergic response to the non-ethanol and/or contaminants in alcoholic beverages, particularly when these beverages are wines or beers. Alcohol-exacerbated rhinitis is more frequent in individuals with a history of rhinitis exacerbated by aspirin.

Aspirin and other non-steroidal anti-inflammatory drugs (NSAIDs), particularly those that inhibit cyclooxygenase 1 (COX1), can worsen rhinitis and asthma symptoms in individuals with a history of either one of these diseases. These exacerbations most often appear due to NSAID hypersensitivity reactions rather than NSAID-induced allergic reactions.

The antihistamine azelastine, applied as a nasal spray, may be effective for vasomotor rhinitis. Fluticasone propionate or budesonide (both are steroids) in nostril spray form may also be used for symptomatic treatment. The antihistamine cyproheptadine is also effective, probably due to its antiserotonergic effects.

A Cochrane review on non-allergic rhinitis reports improvement of overall function after treatment with capsaicin (the active component of chili peppers). The quality of evidence is low, however.

The cause of allergic conjunctivitis is an allergic reaction of the body's immune system to an allergen. Allergic conjunctivitis is common in people who have other signs of allergic disease such as hay fever, asthma and eczema.

Among the most common allergens that cause conjunctivitis are:

- Pollen from trees, grass and ragweed

- Animal skin and secretions such as saliva

- Perfumes

- Cosmetics

- Skin medicines

- Air pollution

- Smoke

- Dust mites

- Balsam of Peru (used in food and drink for flavoring, in perfumes and toiletries for fragrance, and in medicine and pharmaceutical items for healing properties)

- Eye drops

Most cases of seasonal conjunctivitis are due to pollen and occur in the hay fever season, grass pollens in early summer and various other pollens and moulds may cause symptoms later in the summer.

When infants consume peanut proteins while 4 to 11 months old, the risk of developing peanut allergy before the age of 5 years decreases by 11-25%, specifically in children with higher allergy risk via their parents with peanut allergy. From these results, the American Academy of Pediatrics rescinded their recommendation to delay exposure to peanuts in children, also stating there is no reason to avoid peanuts during pregnancy or breastfeeding.

In the case of infectious rhinitis, vaccination against influenza viruses, adenoviruses, measles, rubella, "Streptococcus pneumoniae", "Haemophilus influenzae", diphtheria, "Bacillus anthracis", and "Bordetella pertussis" may help prevent it.

There is conflicting evidence on whether maternal diet during pregnancy has any effect on development of allergies due to a lack of good studies. A 2010 systematic review of clinical research indicated that there is insufficient evidence for whether maternal peanut exposure, or early consumption of peanuts by children, affects sensitivity for peanut allergy.

PKC results from a delayed hypersensitivity/inflammatory reaction to antigens expressed by various pathogens. Common agents include Staph. aureus, Mycobacterium tuberculosis, Chlamydia and Candida.

Some examples:

- Allergic asthma

- Allergic conjunctivitis

- Allergic rhinitis ("hay fever")

- Anaphylaxis

- Angioedema

- Urticaria (hives)

- Eosinophilia

- Penicillin allergy

- Cephalosporin allergy

- Food allergy

- Sweet itch

Atopic reactions are caused by localized hypersensitivity reaction to an allergen. Atopy appears to show a strong hereditary component. One study concludes that the risk of developing atopic dermatitis (3%) or atopy in general (7%) "increases by a factor of two with each first-degree family member already suffering from atopy". As well, maternal stress and perinatal programming is increasingly understood as a root cause of atopy, finding that "...trauma may be a particularly robust potentiator of the cascade of biological events that increase vulnerability to atopy and may help explain the increased risk found in low-income urban populations.”

Environmental factors are also thought to play a role in the development of atopy, and the 'hygiene hypothesis' is one of the models that may explain the steep rise in the incidence of atopic diseases, though this hypothesis is incomplete and in some cases, contradictory to findings. This hypothesis proposes that excess 'cleanliness' in an infant's or child's environment can lead to a decline in the number of infectious stimuli that are necessary for the proper development of the immune system. The decrease in exposure to infectious stimuli may result in an imbalance between the infectious-response ("protective") elements and the allergic-response ("false alarm") elements within the immune system.

Some studies also suggest that the maternal diet during pregnancy may be a causal factor in atopic diseases (including asthma) in offspring, suggesting that consumption of antioxidants, certain lipids, and/or a Mediterranean diet may help to prevent atopic diseases.

The multicenter PARSIFAL study in 2006, involving 6630 children age 5 to 13 in 5 European countries, suggested that reduced use of antibiotics and antipyretics is associated with a reduced risk of allergic disease in children.

A child's allergy is an immune system reaction. The child is reacting to a specific substance, or allergen. The immune system of a child responds to the invading allergen by releasing histamine and other chemicals that typically trigger symptoms in the nose, lungs, throat, sinuses, ears, eyes, skin, or stomach lining. In some children, allergies can also trigger symptoms of asthma—a disease that causes wheezing or difficulty breathing. If a child has allergies and asthma, controlling the allergies is important because the lack of treatment may make the allergies worse. Compounds such as phthalates are associated with asthma in children. Asthma in children is associated with exposure to indoor allergens. in early childhood may prevent the development of asthma, but exposure at an older age may provoke bronchoconstriction. Use of antibiotics in early life has been linked to the development of asthma. Exposure to indoor volatile organic compounds may be a trigger for asthma; formaldehyde exposure, for example, has a positive association.

Treatment usually involves adrenaline (epinephrine), antihistamines, and corticosteroids.

If the entire body is involved, then anaphylaxis can take place, which is an acute, systemic reaction that can prove fatal.

Each home contains possible allergens that can develop into allergies after exposure to:

- Dust mites

- Dogs and cats

- Other furry pets

- Cockroaches

- Mice and rats)

- Plants

- Mold

Vitamin D deficiency at the time of birth and exposure to egg white, milk, peanut, walnut, soy, shrimp, cod fish, and wheat makes a child more susceptible to allergies. Soy-based infant formula is associated with allergies in infants.

Venom from stinging or biting insects such as Hymenoptera (ants, bees, and wasps) or Triatominae (kissing bugs) may cause anaphylaxis in susceptible people. Previous systemic reactions, which are anything more than a local reaction around the site of the sting, are a risk factor for future anaphylaxis; however, half of fatalities have had no previous systemic reaction.

Allergies to a specific pollen are usually associated with OAS reactions to other certain foods. For instance, an allergy to ragweed is associated with OAS reactions to banana, watermelon, cantaloupe, honeydew, zucchini, and cucumber. This does not mean that all sufferers of an allergy to ragweed will experience adverse effects from all or even any of these foods. Reactions may be associated with one type of food, with new reactions to other foods developing later. However, reaction to one or more foods in any given category does not necessarily mean a person is allergic to all foods in that group.

The individual components of atopy are all caused at least in part by allergy (type I hypersensitivity reactions). Therefore, atopic responses appear after the body is exposed to various allergens, for example pollen, dander, dust mites, certain foods, or chemical/physical irritants.

Although atopy has various definitions, in general, it is defined by the presence of elevated levels of total and allergen-specific IgE in the serum, leading to positive skin-prick tests to common allergens.

The majority of children outgrow egg allergy. One review reported that 70% of children will outgrow this allergy by 16 years. In subsequently published longitudinal studies, one reported that for 140 infants who had challenge-confirmed egg allergy, 44% had resolved by two years. A second reported that for 203 infants with confirmed IgE-mediated egg allergy, 45% resolved by two years of age, 66% by four years, and 71% by six years. Children will be able to tolerate eggs as an ingredient in baked goods and well-cooked eggs sooner than under-cooked eggs. Resolution was more likely if baseline serum IgE was lower, and if the baseline symptoms did not include anaphylaxis.

People with atopic diseases such as asthma, eczema, or allergic rhinitis are at high risk of anaphylaxis from food, latex, and radiocontrast agents but not from injectable medications or stings. One study in children found that 60% had a history of previous atopic diseases, and of children who die from anaphylaxis, more than 90% have asthma. Those with mastocytosis or of a higher socioeconomic status are at increased risk. The longer the time since the last exposure to the agent in question, the lower the risk.

OAS produces symptoms when an affected person eats certain fruits, vegetables, and nuts. Some individuals may only show allergy to only one particular food, and others may show an allergic response to many foods.

Individuals with an allergy to tree pollen may develop OAS to a variety of foods. While the tree pollen allergy has been worked out, the grass pollen is not well understood. Furthermore, some individuals have severe reactions to certain fruits and vegetables that do not fall into any particular allergy category. In recent years, it has also become apparent that when tropical foods initiate OAS, allergy to latex may be the underlying cause.

Because the allergenic proteins associated with OAS are usually destroyed by cooking, most reactions are caused by eating raw foods. The main exceptions to this are celery and nuts, which may cause reactions even after being cooked.

Milk allergy typically presents in the first year of life. The majority of children outgrow milk allergy by the age of ten years. One large clinical trial reported resolutions of 19% by age 4 years, 42% by age 8 years, 64% by age 12 years, and 79% by 16 years. Children are be able to tolerate milk as an ingredient in baked goods relative to liquid milk. Resolution was more likely if baseline serum IgE was lower, or if IgE-mediated allergy was absent so that all that was present was cell-mediated, non-IgE allergy.

People with confirmed cow's milk allergy may also demonstrate an allergic response to beef, moreso to rare beef versus well-cooked beef. The offending protein appears to be bovine serum albumin. This is not the same beef allergy that is seen primarily in the southeastern United States, triggered by being bitten by a Lone Star tick.

Milk allergy has consequences. In a U.S. government diet and health surveys conducted in 2007-2010, 6,189 children ages 2-17 years were assessed. For those classified as cow's milk allergic at the time of the survey, mean weight, height and body-mass index were significantly lower than their non-allergic peers. This was not true for children with other food allergies. Diet assessment showed a significant 23% reduction of calcium intake and near-significant trends for lower vitamin D and total calorie intake.

The most common food allergens account for about 90% of all allergic reactions; in adults they include shellfish, peanuts, tree nuts, fish, and egg. In children, they include milk, eggs, peanuts, and tree nuts. Six to 8% of children under the age of three have food allergies and nearly 4% of adults have food allergies.

For reasons not entirely understood, the diagnosis of food allergies has apparently become more common in Western nations recently. In the United States, food allergy affects as many as 5% of infants less than three years of age and 3% to 4% of adults. A similar prevalence is found in Canada.

About 75% of children who have allergies to milk protein are able to tolerate baked-in milk products, i.e., muffins, cookies, cake, and hydrolyzed formulas.

About 50% of children with allergies to milk, egg, soy, peanuts, tree nuts, and wheat will outgrow their allergy by the age of 6. Those who are still allergic by the age of 12 or so have less than an 8% chance of outgrowing the allergy.

Peanut and tree nut allergies are less likely to be outgrown, although evidence now shows that about 20% of those with peanut allergies and 9% of those with tree nut allergies will outgrow them.

In Japan, allergy to buckwheat flour, used for soba noodles, is more common than peanuts, tree nuts or foods made from soy beans.

Corn allergy may also be prevalent in many populations, although it may be difficult to recognize in areas such as the United States and Canada where corn derivatives are common in the food supply.

Food allergies develop more easily in people with the atopic syndrome, a very common combination of diseases: allergic rhinitis and conjunctivitis, eczema, and asthma. The syndrome has a strong inherited component; a family history of allergic diseases can be indicative of the atopic syndrome.

Allergic inflammation is an important pathophysiological feature of several disabilities or medical conditions including allergic asthma, atopic dermatitis, allergic rhinitis and several ocular allergic diseases. Allergic reactions may generally be divided into two components; the early phase reaction, and the late phase reaction. While the contribution to the development of symptoms from each of the phases varies greatly between diseases, both are usually present and provide us a framework for understanding allergic disease.

The early phase of the allergic reaction typically occurs within minutes, or even seconds, following allergen exposure and is also commonly referred to as the immediate allergic reaction or as a Type I allergic reaction. The reaction is caused by the release of histamine and mast cell granule proteins by a process called degranulation, as well as the production of leukotrienes, prostaglandins and cytokines, by mast cells following the cross-linking of allergen specific IgE molecules bound to mast cell FcεRI receptors. These mediators affect nerve cells causing itching, smooth muscle cells causing contraction (leading to the airway narrowing seen in allergic asthma), goblet cells causing mucus production, and endothelial cells causing vasodilatation and edema.

The late phase of a Type 1 reaction (which develops 8–12 hours and is mediated by mast cells) should not be confused with delayed hypersensitivity Type IV allergic reaction (which takes 48–72 hours to develop and is mediated by T cells). The products of the early phase reaction include chemokines and molecules that act on endothelial cells and cause them to express Intercellular adhesion molecule (such as vascular cell adhesion molecule and selectins), which together result in the recruitment and activation of leukocytes from the blood into the site of the allergic reaction. Typically, the infiltrating cells observed in allergic reactions contain a high proportion of lymphocytes, and especially, of eosinophils. The recruited eosinophils will degranulate releasing a number of cytotoxic molecules (including Major Basic Protein and eosinophil peroxidase) as well as produce a number of cytokines such as IL-5. The recruited T-cells are typically of the Th2 variety and the cytokines they produce lead to further recruitment of mast cells and eosinophils, and in plasma cell isotype switching to IgE which will bind to the mast cell FcεRI receptors and prime the individual for further allergic responses.