Annals of Saudi Medicine, Vol 19, No 4, 1999 331 HUMAN SENSITIZATION TO PROSOPIS JULIFLORA ANTIGEN IN SAUDI ARABIA Abdulrahman Al-Frayh, MD, FACAAI; Syed M. Hasnain, PhD, FACAAI(SC); Mohammed O. Gad-el-Rab, MD, FACAAI; Turki Al-Turki, PhD; Khalid Al-Mobeireek, MD; Sultan T. Al-Sedairy, PhD Background: Allergenicity to Prosopis juliflora pollen antigen has been reported from only a few countries, including the US, South Africa, India and Kuwait. In some parts of Saudi Arabia, species of Prosopis have been introduced by the millions as roadside ornamentation. There appear to be four flowering seasons during which pollen grains float in all directions. However, the role of Prosopis pollen as the sensitizing and/or triggering agent of allergic asthma and/or rhinitis in the Kingdom has never been evaluated. Patients and Methods: A total of 473 allergic patients suffering from bronchial asthma in four different geographical regions (Abha, Qassim, Hofuf and Gizan), and attending allergy clinics and chest disease centers of university and Ministry of Health hospitals in the region were tested for immediate hypersensitivity reaction to Prosopis juliflora allergens. Airborne pollen grains at one center were also studied for one full year, using volumetric sampling techniques. Results: A total of 76.1% patients in Qassim, 37.5% in Gizan, 29% in Abha and 11% in Hofuf reacted positively to Prosopis antigen. Multiple sensitivities to other pollen antigens were detected in all patients. The level of airborne Prosopis pollen detected in Gizan exceeded 90 grains m -3 of air. Conclusion: In view of the documented evidence of Prosopis-involved allergenicity, the role of Prosopis pollen as a sensitizing factor in Saudi Arabia has been confirmed. However, the cause of elicitation of symptoms in many multiple sensitive patients, together with the question of cross-reactivities, needs thorough and detailed investigation. In vitro confirmation of all positive results is also required to incriminate Prosopis as one of the major allergens in parts of Saudi Arabia. Ann Saudi Med 1999;19(4):331-336. Key Words: Prosopis juliflora, allergenicity. Allergenicity to Prosopis juliflora pollen antigen has been reported from a few countries, including the United States, 1,2,3 Kuwait, 4 India 5,6,7 and South Africa. 8 Several species of the drought-resistant genus Prosopis have been introduced to Saudi Arabia as roadside ornamentation. Many roads and streets in several cities of the Kingdom have a large number of trees belonging to the genus Prosopis. Prosopis spp. are a native of South and Central America and India, and are closely related to Acacia, a known allergenic plant genus. Out of the estimated 44 species of Prosopis, only nine are known to be present in the Kingdom. These are Prosopis alba, P. chilensis, P. glandulosa, P. juliflora From the College of Medicine and King Khalid University Hospital (Drs. Al- Frayh, Gad-el-Rab, and Al-Mobeireek), King Abdulaziz City for Science and Technology (Dr. Al-Turki), and the Department of Biological and Medical Research (Drs. Hasnain and Al-Sedairy), King Faisal Specialist Hospital and Research Centre, Riyadh, Saudi Arabia. Address reprint requests and correspondence to Dr. Al-Frayh: Department of Pediatrics, College of Medicine, King Saud University, P.O. Box 7805, Riyadh 11472, Saudi Arabia. Accepted for publication 25 April 1999. Received 5 May 1997. (also known as mesquite), P. spicigera (or P. cineraria), P. tamarugo, P. velutina, P. farcta, and P. pallida. Among these, P. farcta and P. spicigera are considered to have existed for a long time in the Arabian Peninsula. 9,10 P. juliflora is known to flower about four times a year in the region, and during the period large amounts of pollen debris are deposited underneath trees. These are easily distributed by moving vehicles as well as by human and animal activities. Individuals walking on the roadside are also likely to collect pollen on shoes and clothing and transfer them to indoor environments. Published data from different countries indicate that Prosopis pollen, which pollinates partly by insects, can become airborne, 2,4,6 and can be trapped and identified on slides from air samplers. Airborne pollen of Prosopis can be inhaled through the nose and/or mouth as other pollen, resulting in sensitization of susceptible people and subsequent elicitation of symptoms of respiratory allergic diseases. 7,8 Because of the number of trees in Saudi Arabia, the role of Prosopis pollen as an airborne triggering agent for the elicitation of allergic symptoms has been a subject of