This review presents state-of-the-art knowledge and identifies knowledge gaps for future research in the area of exercise-associated modifications of infection susceptibility. Regular moderate-intensity exercise is believed to have beneficial effects on immune health through lowering inflammation intensity and reducing susceptibility to respiratory infections. However, strenuous exercise, as performed by professional athletes, may promote infection: in about half of athletes presenting respiratory symptoms, no causative pathogen can be identified. Acute bouts of exercise enhance the release of pro-inflammatory mediators, which may induce infection-like respiratory symptoms. Relatively few studies have assessed the influence of regularly repeated exercise on the immune response and systemic inflammation compared to the effects of acute exercise. Additionally, ambient and environmental conditions may modify the systemic inflammatory response and infection susceptibility, particularly in outdoor athletes. Both acute and chronic regular exercise influence humoral and cellular immune response mechanisms, resulting in decreased specific and non-specific response in competitive athletes. The most promising areas of further research in exercise immunology include detailed immunological characterization of infection-prone and infection-resistant athletes, examining the efficacy of nutritional and pharmaceutical interventions as countermeasures to infection symptoms, and determining the influence of various exercise loads on susceptibility to infections with respiratory viruses, including SARS-CoV-2. By establishing a uniform definition of an "elite athlete," it will be possible to make a comparable and straightforward interpretation of data from different studies and settings.

Allergy and Clinical Immunology Research Group Department of Microbiology Immunology and Transplantation KU Leuven Leuven Belgium

Allergy and Immunology Centre Tbilisi Georgia

Allergy Unit Hospital CUF Descobertas Lisbon Portugal

Basic and Clinical Immunology Department of Pathology Faculty of Medicine University of Porto Porto Portugal

Department of Cardiovascular and Thoracic Sciences Fondazione Policlinico Universitario A Gemelli IRCCS Università Cattolica del Sacro Cuore Rome Italy

Department of Immunology and Allergy Medical University of Łódź Łódź Poland

Department of Medical Sciences and Public Health M Aresu University of Cagliari Cagliari Italy

Department of Respiratory Medicine 1st Faculty of Medicine Charles University and Thomayer Hospital Prague Czech Republic

Department of Respiratory Medicine and Allergology Institute for Clinical Science Skane University Hospital Lund University Lund Sweden

Epidemiology Research Unit Instituto de Saúde Pública University of Porto Porto Portugal

National Heart and Lung Institute Imperial College London London UK

Serviço de Imunoalergologia Centro Hospitalar de São João E P E Porto Portugal

Teaching University Geomedi LLC Tbilisi Georgia

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Complement System Deficiencies in Elite Athletes

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