Immune fitness, defined as the adequate functioning of the immune system, is essential for maintaining health, preventing and resolving disease, and improving quality of life. The immune status questionnaire (ISQ) is a self-assessment tool developed to evaluate the type and frequency of seven common immune-related complaints. This review was conducted to provide a comprehensive overview of studies that utilized the ISQ and provide recommendations for its future improvement. A literature search on Google Scholar, supplemented by cross-referencing, identified 91 publications, of which 51 (from 38 independent studies) reported ISQ data and were included in this review. These studies were conducted in 14 countries, with the ISQ translated into six languages. They were cross-sectional and experimental studies involving healthy subjects or patient populations, with sample sizes ranging from 22 to 45,782 participants. Findings revealed that ISQ scores significantly differed across demographics (e.g., sex and age) and health status (e.g., obesity and diabetes). Significant correlations were found between ISQ scores and various health correlates, including positive correlations with lifestyle factors – such as regular physical activity, attaining a daily diet, and adequate sleep – and negative correlations with mood states such as anxiety and depression, alcohol use, hangovers, and smoking. In addition, the ISQ scores also showed associations with immune biomarkers indicative of systemic inflammation. The review also discusses the strengths and limitations of the ISQ, including its 12-month recall period, which may be adapted in repeated-measures designs. Potential improvements to the ISQ include incorporating additional characteristics such as duration, severity, and impact of immune-related complaints and refining the current scoring method. In conclusion, the ISQ is a simple, cost- and time-effective questionnaire to assess immune fitness. The ISQ can be successfully implemented in research and screening in clinical settings.