The immune system evolved to recognise infectious pathogens. Discrimination between self and nonself is a critical property of the immune system that enables it to eradicate pathogens without harming the host. Immune responses to pathogens are affected initially by the innate immune system, which reacts rapidly to generic pathogen-associated molecules and primes the adaptive immune system, which is slower to react but has exquisite specificity and memory. Here, we outline the spatial framework of the immune system and define its mobile and fixed elements. A range of cell types (including antigen-presenting cells and lymphocytes) and soluble components (including complement, antibody, cytokine and chemokine proteins) circulate and communicate between the bone marrow and lymphoid tissue structures to maintain immune surveillance.

Key Concepts:

• The immune system is designed to recognise, react to and eradicate ‘nonself’, in particular infectious pathogens.
  
• The immune system comprises fixed and mobile elements.
  
• The elements of the immune system sequentially mediate innate immunity, providing acute, generic defences and adaptive immunity, providing highly specific, long-lasting defences.
  
• The specificity and diversity of adaptive immunity is based on genetic polymorphism of the receptors for antigens on T and B cells and of the major histocompatibility complex (MHC) molecules that bind antigenic peptides recognised by the T-cell receptor.
  
• Nonspecific costimulation via surface-interacting molecules or soluble cytokines and chemokines shapes the differentiation of T and B cells in the adaptive immune response.