Food Allergies Lecture 26 April 26, 2010 Dr. Ponnusamy
Prevalence of food allergy
Experts agree that allergies in developed countries are becoming more common.
In the U.S., food allergies afflict 2-2.5% adults and 6-8% children.
100-175 people in the U.S. die each year.
Death generally result from anaphylactic shock, often to peanuts or tree nuts.
More than 160 foods have been associated with allergic reactions.
What is food allergy?
Food allergy is an inappropriate immune response to an otherwise harmless food.
True food allergy involves several types of immunological responses.
Food allergens are usually proteins.
Some foods may contain haptens or haptens carrier.
Understanding Immunological concepts
Human body has many defense mechanisms to fight off infectious diseases and other toxic foreign substances.
Strong healthy adult human can fight off most of infectious diseases.
Ability to fight off disease can be modulated by genetics, age, race and lifestyles (diets, exercise and amount of sleep etc.)
Allergic reactions are Antigen-Antibody reactions
Antigen = a foreign substance
Antibody = a protein produced in response to an antigen that is capable of binding specifically to the antigen!
Haptens - a small molecule that has the ability to combine with an Ab or a cell-surface receptor.
Human body has two categories of defense system
Non specific defenses
Physical barriers (skin and mucous membrane)
Chemical barriers (saliva, mucus, gastric juices, etc)
Cellular defenses (certain cells can eat invaders-phagocytes)
Inflammation (reddening, swelling and temperature increase of the affected sites)
Fever (elevated body temperature)
Molecular defenses (interferons or complementary system etc.)
Specific defenses or specific immunity**
Antibodies (many kinds of antibodies for many kinds of antigens)
Food allergies are related to specific defenses or specific immunity.
Immune literary means “free of burden”.
Actions of the immune system are triggered by antigens (foreign substances).
Most antigens are large protein molecules ; Some antigens are polysaccharides and few are glycoproteins (carbohydrate and protein) or nucleo-proteins .
Specific Immunity Immunity Innate (inborn) Genetic factors Acquired Active (own Ab) Passive (Ready-made-Ab) Natural (Exposure to Foreign Agents) Artificial (immunization) Natural Maternal Ab Ab = Antibodies Artificial (Ab from Other sources)
Produces by B-linfocytes (bone marrow)
Five classes of Immunoglobulins
1) IgG: Main class of antibodies in blood-also from mother-to-child (20%)
2) IgA: Small amount in blood, but larger amount in tears, milk, saliva, mucus and the lining tissues
3) IgM: First Antibody secreted during the primary response
4) IgE: Found mainly in body fluids and skin --- Associated with allergy reactions!
5) IgD: Found in B-Cell membrane
Antibody structure Binding site for the antigen Binding site for the receptor
Primary and secondary responses to an antigen Primary response : first response when host’s B-cell recognize the antigen Secondary response : upon second exposure to the antigen, the Memory cells will divide, thus make more of the total antibody
Nature of IgE Allergic Reactions Antigen + IgE + Mast cells = Mediator release Mediators= histamine and others Picture credit: from Dr. Gary E. Kaiser http://www.cat.cc.md.us/courses/bio141/lecguide/index.html
What is Histamine? Histamine triggers the inflammatory response . Histamine is produced by basophils ( a type of white blood cell) and by mast cells ( a resident cell in tissues of the body; present more in skin, lungs, eyes, digestive tract, mouth, nose and conjunctiva) in response to antigens. Found in almost all animal body cells, histamine increases the permeability of the capillaries to white blood cells and other proteins, in order to allow them to engage foreign invaders in the affected tissues.
What does histamine do?
Allergic reactions are mediated by the release of mediators such as histamine
Vasodilation, increased capillary permeability, bronchoconstriction etc.
Types of food allergies
Immediate hypersensitivity with IgE which occurs within minutes to a few hours after ingestion of offending foods.
Systemic : Itching, urticaria (hives), Vomiting, Abdominal cramps, diarrhea and respiratory distress, and in severe cases anaphylactic shock
Localized : hives and eczema or atopy (an umbrella term covering clinical presentations of food allergy etc)
Delayed hypersensitivity reactions (>8hours after ingestion): cellular immunity involving T-lymphocytes and macrophages
B lymphocytes – mature in B one marrow and lymphatic tissue (spleen and lymph nodes) T lymphocytes – mature in the T hymus Lymphocytes
Recognise antigens on surface of leukocytes, especially macrophages
Enlagre and form a clone of T-helper cells
Secrete interferon and cytokines which stimulate B-cells and stimulate killer -cells
Can be infected by HIV
Also called cytotoxic
Destroy abnormal body cells, e.g. virus infected or cancer cells
Stimulated by cytokines (THcells)
Release perforin , which forms pores in target cells. This allows water and ions in = lysis
Control the immune system when the antigen /pathogen has
Only recently discovered so
little is known about them
Can survive a long time and give lifelong immunity from infection
Can stimulate memory B-cells to produce antibodies
Can trigger production of killer T cells
Mature in Thymus, which is most active just before and after birth. The thymus starts to shrink during puberty.
How T-cells work… Abnormal cell e.g cancer cell, infected cell Normal cell Antigen Killer T-cell recognises antigen Clones of killer T-cell attach to antigen Helper T-cell stimulates correct killer T-cell to multiply Killer T-cells release perforin pores Abnormal cell gains water, swells and bursts Helper T-cell also stimulates B-cells to make antibodies Memory T-cells stay in circulation Suppressor T-cells turn off immune response X X X
Stages of food allergy or hypersensitivity
A. Sensitization: initial meeting of an allergen and the immune system that results in IgE production!
B. Activation of mast cells
IgE : is specific for a particular allergen that triggers the allergic reaction. The allergen binds to the immunoglobulin on specific immune cells called basophils and mast cells. This binding results in the release of chemicals that cause inflammation in the body, such as histamine, within 30 minutes of exposure. These chemical mediators cause allergy symptoms, such as urticaria (hives), runny nose, watery eyes, sneezing, wheezing and itching.
Non-IgE substances (eg. Drugs)
Most common sites for allergic reactions
* Exercise exacerbates symptoms
Anaphylaxis Potential fatal reaction to a food allergen causing reduced oxygen supply to the heart and other body tissues. Symptoms include: difficult breathing, low blood pressure, pale skin, a weak rapid pulse, loss of consciousness, death.
Most common allergenic foods
Legumes (Peanuts and Soybeans)
Mollusks and shellfish (snails, mussels, oysters, scallops, clams, squid)
Fish (cod, salmon, haddock etc)
Crustacea (shrimp, crawfish, lobster etc.)
Tree nuts (almonds, walnuts, Brazil nuts etc)
Selected food additives
Children and Food Allergy
High prevalence in the first few years of life
Immature immune system and immature digestive system
Cow’s milk and soy, most common allergens
Breast feeding is recommended
Delay introduction of common allergenic foods
Sensitivity to most of the allergenic foods is usually lost by young children as they grow up
Cross-Reaction: the reaction of one antigen with antibodies developed against another antigen.
Hidden food ingredients in ready made food products
Milk and milk product derivatives
Egg and egg derivatives
Peanuts, tree nuts and derivatives
Fish derivatives (surimi, fish sauce, fish paste etc)
Soy and its derivatives
Eating out is a nightmare?
African, Chinese, Indonesian, Mexican, Thai, and Vietnamese dishes often contain peanuts. It is recommended that peanut-allergic individuals avoid these types of foods and restaurants.
For traditional food restaurants, cross-contamination of allergens to other foods can also be a problem.
People who have food allergy need a total avoidance of the offending foods.
Read food ingredient list.
Eliminate cross-contamination during cooking and preparation!!!!
Common prescribed medications
epinephrine (relaxes smooth muscle, constricts blood vessels, and stimulates the heart; used for severe systemic reactions-anaphylaxis);
antihistamines (block the binding of histamine to histamine receptors on target cells);
sodium cromolyn (prevents mast cells from releasing histamines).
Food protein proctocolitis/ proctitis (inflammation of rectum)
Rectum is the final portion of large intestine. It empties stools from body through anus
Food protein-induced enteropathy : Celiac disease , an adverse immune response to the protein gluten.
Milk-soy protein intolerance (MSPI): non-IgE mediated allergic response to milk and/or soy protein during infancy and early childhood. Symptoms of MSPI are usually attributable to food protein proctocolitis or FPIES.
Heiner syndrome - lung disease due to formation of milk protein/IgG antibody immune complexes (milk precipitins) in the blood stream after it is absorbed from the GI tract. The lung disease commonly causes bleeding into the lungs and results in pulmonary hemosiderosis.
A strong desire to avoid a particular food Food Aversion Adverse reaction to food that does not involve the immune system Food Intolerance
Direct effect of food
Enzyme deficiency (e.g., lactase, sucrase etc)
Symptoms of food intolerance: bloating, cramping, gas and diarrhea
Main cause of food intolerance: carbohydrates (lactose, fructose, sorbitol)
Allergy VS Intolerance
True Allergy-Total avoidance necessary!
Intolerance- Small amount may be tolerated
Other causes of allergy-like food problems
Microbial products- e.g. histamine – Some food products have high levels of histamine (eg fermented foods)
Diagnosis Determine if the symptoms are mediated by the immune system Complete physical Detailed case history Food diary Positive identification of the allergen
Prick skin Test (PST)
Drop of the substance under test on the forearm
Allow a tiny amount to enter the skin (doctor pricks it with a needle)
After 15min, verify presence of “bump”
Radioallergosorbent Test (RAST)
Requires blood samples
Laboratory test are performed to look if the patients has IgE against specific types of food
It is a radioimmunoassay test
A person who has outgrown an allergy may still have a positive IgE years after exposure.
The suspected allergen is bound to an insoluble material and the patient's serum is added. If the serum contains antibodies to the allergen, those antibodies will bind to the allergen. Radiolabeled anti-human IgE antibody is added where it binds to those IgE antibodies already bound to the insoluble material. The unbound anti-human IgE antibodies are washed away. The amount of radioactivity is proportional to the serum IgE for the allergen.
Resources for food allergies
Food Allergy and Anaphylaxis network (FAAN)
American Academy of Allergy, Asthma and Immunology