The human immune system

The human body is frequently exposed to harmful challenges, both from external sources (such as viruses, bacteria, fungi, and parasites, also called pathogens) and internal sources (such as tumours). The immune system provides several layers of defence to protect against these challenges, and when successful, a state of immunity is established (Latin: immunitas, freedom from). However, the robustness and regulation of this response are influenced by many factors, including exercise, as outlined in the remaining chapters of this book. The present chapter provides a concise overview of the human immune system components that are most relevant to exercise immunology. The focus will be on the characteristics and functions of different cells of the immune system and other soluble components that contribute to the mechanisms giving rise to a whole-body immune response.