**Abstract**

Probiotics have emerged as an in-demand and highly marketed commodity in the healthcare space. In 2021, the global market valued the probiotic industry at USD 58.17 billion in 2021. It is expected to have a compound annual growth rate of 7.5% yearly from 2021 to 2030. The inclusion of probiotics in various products has become synonymous with health benefits despite limited understanding of mechanism of action or benefit. This chapter will survey the state of our understanding of the interactions between probiotics with the innate immunity, adaptive immunity, and the host gut microbiome. Additionally, we will also highlight the theorized beneficial and possible detrimental immunomodulatory effects of probiotics on human health.

**Keywords:** probiotics, adaptive immunity, innate immunity, microbiome, clinical use of probiotics
