III ALLERGIC CONJUNCTIVITIS
Mohamed T. Higazy
The allergic response is considered to be an over-reaction of the body’s immune system to foreign substances (allergens). The response can be innate or acquired. The key component to the ocular allergic response is the mast cell. When mast cells interact with specific allergens,
the process causes a discharge of chemical mediators into the surrounding tissues. The prima- ry chemical mediator released during degranulation is histamine, which is responsible for
increased vascular permeability, vasodilation, bronchial contraction and increased secretion of mucus. Heparin, chymase, tryptase and other substances are also released from mast cells. In severe or prolonged allergic reactions, a “late-phase” response may occur in which cell mem- branes begin to break down into arachidonic acid, which is further degraded to form pro-
staglandins, leukotrienes and thromboxane (powerful mediators of inflammation that initia- te stimulation of pain receptors and migration of white blood cells). 
Seasonal allergic conjunctivitis (SAC) and perennial allergic conjunctivitis (PAC) are classical examples of the “early-phase” response; however, the “late-phase” response includes atopic keratoconjunctivitis (AKC), and vernal keratoconjunctivitis (VKC).
Allergic conjunctivitis is an allergic inflammatory response in the conjunctiva. Approximately 70 % of patients with allergic conjunctivitis have an associated atopic disease, such as allergic rhinitis, asthma, or atopic dermatitis.
In most reports allergic conjunctivitis affects approximately 15 to 20 % of the world’s popula- tion.  This incidence appears to be increasing due to increased air pollution and cigarette smoke, which may be responsible for the increased sensitivity to allergens.  Given the high incidence of allergic conditions, it is very common for patients suffering from allergic con-
junctivitis to present to general practitioners or general ophthalmologists. Correct diagnosis and treatment is essential to manage such patients.
CLASSIFICATION AND CLINICAL PICTURE OF ALLERGIC CONJUNCTIVITIS
According to Bonini and Bonini (1997) allergic conjunctivitis is classified into: 
Seasonal allergic conjunctivitis – Acute (SAC)
Perennial allergic conjunctivitis – Chronic (PAC)
Vernal keratoconjunctivitis (VKC)
Atopic keratoconjunctivitis (AKC)
Giant papillary conjunctivitis (GPC)
The hallmark of allergic conjunctivitis is itching. A patient having red itchy eyes with no palpable preauricular lymph nodes is most probably having allergy.
SEASONAL ALLERGIC CONJUNCTIVITIS (SAC) & PERENNIAL ALLERGIC CONJUNCTIVITIS (PAC) Common airborne antigens including pollen, grass, and weeds may provoke the symptoms of acute allergic conjunctivitis in the form of ocular itching, redness, burning, and tearing. The
main distinction between seasonal and perennial allergic conjunctivitis is the timing of symp- toms.
Individuals with SAC typically have symptoms of acute allergic conjunctivitis during a defined period of time; in spring when the predominant airborne allergen is tree pollen; in summer when the predominant allergen is grass pollen; or in autumn when the predominant allergen is weed pollen. Typically, persons with SAC are symptom-free during the winter months in cooler climates because of the decreased airborne transmission of these allergens.
In contrast, individuals with PAC may have symptoms that last the whole year; thus, PAC may not be caused exclusively by seasonal allergens, although, they may
play a role. Common household allergens such as dust mite, cockroaches, and pet dander are the usual causes of PAC.
Classic signs of allergic conjunctivitis include injection of conjunctival vessels as well as varying degrees of chemo- sis (conjunctival oedema) and eyelid oedema. The conjunctiva often has a milky appearance (Figure 1) due to obscuration of superficial blood vessels by oede- ma within the substantia propria of the conjunctiva. Oedema is generally believed to be the direct result of increased vascular permeability caused by release of histamine from conjunctival mast cells. Figure 1 – Milky appearance of bulbar conjunctiva