Risk factors for allergies can be placed in two broad categories:
host and
environmental factors. and can trigger allergic reactions such as asthma,
eczema, or
itching. The mite's gut contains potent
digestive enzymes (notably
peptidase 1) that persist in their faeces and are major inducers of allergic reactions such as
wheezing. The mite's
exoskeleton can also contribute to allergic reactions. Unlike
scabies mites or skin
follicle mites, house dust mites do not burrow under the skin and are not parasitic. Dust mite-proof encasements to
mattress,
pillow, and
duvet prevent chronic contact with allergens.
Foods A wide variety of foods can cause allergic reactions, but 90% of allergic responses to foods are caused by cow's milk,
soy,
eggs,
wheat,
peanuts,
tree nuts, fish, and
shellfish. Other food allergies, affecting less than 1 person per 10,000 population, may be considered "rare". Although
peanut allergies are notorious for their severity, peanut allergies are not the most common food allergy in adults or children. Other allergens may trigger severe or life-threatening reactions and are more common when combined with
asthma. The sensitivity is usually to proteins in the
white, rather than the
yolk. Approximately 60% of milk-protein reactions are
immunoglobulin E–mediated, with the remaining usually attributable to
inflammation of the colon. Some people are unable to tolerate milk from goats or sheep as well as from cows, and many are also unable to tolerate dairy products such as cheese. Roughly 10% of children with a milk allergy will have a reaction to beef. Lactose intolerance, a common reaction to milk, is not a form of allergy at all, but due to the absence of an
enzyme in the
digestive tract. Those with
tree nut allergies may be allergic to one or many
tree nuts, including
pecans,
pistachios, and
walnuts.
Latex Latex can trigger an IgE-mediated cutaneous, respiratory, and systemic reaction. The prevalence of latex allergy in the general population is believed to be less than one percent. In a hospital study, 1 in 800 surgical patients (0.125 percent) reported latex sensitivity, although the sensitivity among healthcare workers is higher, between seven and ten percent. Researchers attribute this higher level to the exposure of healthcare workers to areas with significant airborne latex allergens, such as operating rooms, intensive-care units, and dental suites. These latex-rich environments may sensitize healthcare workers who regularly inhale allergenic proteins. These people often have
perioral itching and local
urticaria. Only occasionally have these food-induced allergies induced systemic responses. Researchers suspect that the cross-reactivity of latex with banana,
avocado,
kiwifruit, and
chestnut occurs because latex proteins are structurally
homologous with some other plant proteins. injects saliva proteins into the human bloodstream, which can cause allergy.
Insect stings One of the main sources of human allergies is insects. Insect bites, stings, ingestion, and inhalation can trigger an insect allergy.
Toxins interacting with proteins Another non-food protein reaction,
urushiol-induced contact dermatitis, originates after contact with
poison ivy,
eastern poison oak,
western poison oak, or
poison sumac.
Urushiol, which is not itself a protein, acts as a
hapten and chemically reacts with, binds to, and changes the shape of
integral membrane proteins on exposed skin cells. The immune system does not recognize the affected cells as normal parts of the body, causing a
T-cell-mediated
immune response. Of these poisonous plants, sumac is the most virulent. The resulting dermatological response to the reaction between urushiol and membrane proteins includes redness, swelling,
papules,
vesicles,
blisters, and streaking. Estimates vary on the fraction of the population that will have an immune system response. Approximately 25% of the population will have a strong allergic response to urushiol. In general, approximately 80–90% of adults will develop a rash if they are exposed to of purified
urushiol. Some people are so sensitive that a molecular trace on the skin can initiate an allergic reaction.
Genetics 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.
Ethnicity may play a role in some allergies; however, racial factors have been difficult to separate from environmental influences and changes due to
migration. Multiple studies have investigated the genetic profiles of individuals with predispositions to and experiences of allergic diseases, revealing a complex polygenic architecture. Specific genetic loci, such as MIIP, CXCR4, SCML4, CYP1B1, ICOS, and LINC00824, have been directly associated with allergic disorders. It is an important cytokine for many steps in B-cell maturation and differentiation, since it increases CD23 and MHC class II molecules, and aids in B-cell isotype switching to IgE. The more striking thing is that IL-13 is the prime mover in allergen-induced asthma via pathways that are independent of IgE and eosinophils. In other words, individuals living in too sterile an environment are not exposed to enough pathogens to keep the immune system busy. Since our bodies evolved to deal with a certain level of such pathogens, when they are not exposed to this level, the immune system will attack harmless antigens. Thus, normally benign microbial objects—like pollen—will trigger an immune response. The hygiene hypothesis was developed to explain the observation that
hay fever and
eczema, both allergic diseases, were less common in children from larger families, which were, it is presumed, exposed to more infectious agents through their siblings, than in children from families with only one child. It is used to explain the increase in allergic diseases that have been seen since industrialization, and the higher incidence of allergic diseases in more developed countries. The hygiene hypothesis has now expanded to include exposure to symbiotic bacteria and parasites as important modulators of immune system development, along with infectious agents. Epidemiological data support the hygiene hypothesis. Studies have shown that various immunological and autoimmune diseases are much less common in the developing world than in the industrialized world, and that immigrants to the industrialized world from the developing world increasingly develop immunological disorders in relation to the length of time since arrival in the industrialized world.
Other environmental factors Allergic diseases are more common in industrialized countries than in countries that are more traditional or agricultural, and there is a higher rate of allergic disease in urban populations versus rural populations, although these differences are becoming less defined. Historically, the trees planted in urban areas were predominantly male to prevent litter from seeds and fruits, but the high ratio of male trees causes high pollen counts, a phenomenon that horticulturist Tom Ogren has called "
botanical sexism". Alterations in exposure to
microorganisms is another plausible explanation, at present, for the increase in
atopic allergy. In particular, research suggests that allergies may coincide with the delayed establishment of
gut flora in
infants. It may be that the term 'parasite' could turn out to be inappropriate, and in fact a hitherto unsuspected
symbiosis is at work. ==Pathophysiology==