Immunity: Humoral and Cellular


The immune system evolved to recognise and combat infectious pathogens. The ability to discriminate between normal self and nonself is a critical property of the immune system which enables it to eradicate pathogens without causing autoimmune disease and harming the host. The first line of defence, the older ‘innate’ immune system of cells (granulocytes, monocytes–macrophages, natural killer and other innate lymphoid cells) and soluble mediators (cytokines, chemokines and complement factors), provides a rapid, short‐lived response to a broad range of generic pathogen‐associated molecules. Innate immunity primes the second line of defence, the ‘adaptive’ immune system, first present in jawed vertebrates, comprising lymphocytes (T cells and antibody‐producing B cells) with highly specific antigen receptors and antigenic memory. Dendritic cells are a type of innate immune cell that links innate and adaptive immunity by their ability to present antigens to T cells. The spatial framework of the immune system with its mobile and fixed elements is described herein.

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 a rapid, broad defence and adaptive immunity, providing highly specific, long‐lasting defences.
  • The exquisite specificity and diversity of adaptive immunity is based on postgenetic, somatic hypermutation‐ and recombination‐based 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 T‐cell receptors.
  • Nonspecific costimulation via surface‐interacting molecules or soluble cytokines and chemokines shapes the differentiation of T and B cells in the adaptive immune response.
  • Immune responses are highly context‐dependent, being influenced by the local immune and nonimmune environment, and the epigenetic regulation of immune genes.

Keywords: innate; adaptive; antigen; MHC; T cell; B cell; antibody; cytokine; chemokine

Figure 1. Interactions between the antigen‐presenting cell (APC) and the T cell. Antigen peptide (Ag) processed by the APC is presented on the major histocompatibility complex (MHC) molecule to the cognate T‐cell receptor (TCR). This contact is stabilised by intercellular adhesion molecule (ICAM) 1–leucocyte functional antigen (LFA)‐1, LFA‐3–CD2 and MHC–CD4 or MHC–CD8 interactions. In addition to stimulation via the highly specific MHC–Ag–TCR interaction, nonspecific costimulation occurs via B7–CD28. This is essential for proper activation of the responding T cell, without which it may become subsequently unresponsive (anergic). Cytokines secreted by the APC direct T‐cell differentiation. Upregulation of CD40 ligand (CD40L) on the T cell further stimulates the APC to present antigen and secrete cytokines. T‐cell proliferation is limited by the upregulation of CTLA‐4, which inhibits costimulation mediated by CD28. PD‐1 and its ligands are members of the CD28/CTLA‐4 family of T‐cell regulators that have a general effect to negatively regulate T‐cell responses.
Figure 2. Structure of an IgG antibody molecule. Reproduced from Lucas AH () © Wiley.


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Harrison, Leonard C(Apr 2016) Immunity: Humoral and Cellular. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001236.pub3]