The immune system is exceedingly complex in its constituent cells, molecules, and signaling pathways. Each major component of the immune system is critical for survival; immune activity protects against infections that would quickly be lethal without immune defenses and eliminates cells in the stages of cancerous transformation.

The most common and major immune system disorders are related to an immune activity that exceeds physiological needs. Hypersensitivity in the form of allergies occurs in 10% to 20% of the population. The prevalence of allergies increased in the developed world from the 1960s through the early 2000s, after which it began to plateau. Although less common than immune hyperactivity, disorders in which immune activity is below normal leave an individual susceptible to dangerous infections. In some individuals, immune activity is compromised to the extent that those affected are at risk for a major illness or even death.