Allergy

Allergy is also known as a Type I Hypersensitivity reaction, Immediate Hypersensitivity reaction or Atopy. Allergies occur when the body perceives a relatively harmless substance as a threat and activates the immune system to deal with this perceived threat. These reactions are mediated by IgE antibody and mast cells, and followed by inflammation. Allergies are present in atopic individuals (persons with a genetic tendency towards allergies).

Sequence of events in the development of an allergy:
1) Production of IgE antibodies in response to antigen with the assistance of T helper cells. T helper cells secrete cytokines which stimulates B lymphocytes specific to the antigen to proliferate and produce IgE.
2) IgE antibodies produced are specific to the antigen and bind to Fc receptors of Mast cells in a process called sensitization.
3) When antigen is re-intoduced, there’s cross-linking of IgE antibodies bound on the mast cells by the antigen.
4) When a critical mass of IgE antibodies on a mast cell has been cross-linked, the mast cell gets activated and release mediators like histamines, prostaglandins and leukotrienes (which cause vasodilation, smooth muscles contraction and increases vascular permeability; histamines increase mucus secretion), proteases (which damage local tissues and cause inflammation), and cytokines (which stimulates recruitment of leukocytes and prolongs inflammation).

NB: An allergen is a substance that causes an allergy, i.e. the antigen that leads to the development of an allergy.

Now that we know what an allergy is, we can better understand what hay fever and eczema are all about.

Hay Fever

It is also known as allergic rhinitis or inflammation of the nasal mucosa. It is a misnomer since hay is not the usual cause of the problem and it also does not cause fever. It is an allergic reaction to inhaled allergens, most commonly pollen from weeds, grasses and trees. Other common allergens include animal protein, dust mites and pores from mold. A point to note is that any substance can be an allergen as long as it triggers an allergic response in the individual. The ones listed are the more common ones. Signs and symptoms are a result of the mediators released by mast cells activated by the allergen. They include nasal congestion, clear running nose, swelling and sneezing. Allergic rhinitis can often lead to sinusitis (inflammation of the paranasal sinuses or air cavities within the passages of the nose), allergic conjunctivitis and asthma. Allergens that cause hay fever often travel up the upper respiratory tract and sensitize mast cells present in the bronchial tubes, priming the immune system. A re-exposure to the allergen or to other asthmatic triggers like exercise and irritants can cause an asthma episode.

Eczema

Eczema is a general term for many types of skin inflammation or dermatitis. The most common form is atopic dermatitis which is what we are interested in as it is an allergic disorder and associated with asthmatic patients.

Atopic dermatitis refers to the chronic inflammation of skin and is characterized by dry, itchy, inflamed skin. It often occurs in people with a family history of allergic conditions i.e. asthma and hay fever, and is most commonly found in infants and children. It is not contagious. At present, the exact cause is not known but evidence points towards genetic and environmental factors. It is thought that the sensitivity of the skin is passed down through genes and that exposure of allergens and/or irritants to the super sensitized skin results in the disease. Common allergens include pollen, animal dander (tiny particles from skin or hair) dust mites or pores from mold. Irritants include dust, sand, cigarette smoke, wool or synthetic fibers, soaps, perfumes, cosmetics. A point to note: it is often difficult to tell if the basis of the disease is allergic or irritant as irritants produce the same symptoms as allergens. The differentiation between the two is that allergens are specific to B lymphocytes and causes an increase in IgE antibodies while irritants are non-specific and there is no rise in IgE levels. Signs and symptoms include dry, scaly patches of skin; papules which are small, raised bumps that may open when scratched becoming crusty and infected; hives which are red, raised bumps; Lichenification or thick, leathery skin resulting from constant scratching and rubbing; Hyperlinear palms; Keratosis pilaris: small, rough bumps, generally on the face, upper arms, and thighs.



End-Note
Asthma, hay fever and atopic dermatitis are all atopic disorders i.e. type I hypersensitivity. They are often associated with one another due to their similar etiologies. A patient with asthma would probably have a history of hay fever or atopic dermatitis.

Contributed by John Lee