Immune system

The immune system is the integrated biological defence network of an organism, comprising cells, proteins, tissues, and organs that collectively identify and eliminate pathogens, foreign molecules, and dysfunctional host cells. Found in virtually all multicellular life forms, the immune system ranges from simple antimicrobial peptides in invertebrates to the highly sophisticated adaptive immune responses of vertebrate animals, including humans.

Components of the immune system

The immune system is broadly divided into two interconnected arms: the innate immune system and the adaptive immune system. These two arms communicate through chemical signals known as cytokines and through direct cell-to-cell contact.

Innate immune system

The innate immune system provides the first line of defence against infection. It responds rapidly — typically within minutes to hours — but does not retain immunological memory of specific pathogens. Key components include:

• Physical and chemical barriers: skin, mucous membranes, stomach acid, and antimicrobial peptides such as defensins.
• Phagocytic cells: macrophages and neutrophils engulf and destroy pathogens through a process called phagocytosis.
• Natural killer (NK) cells: innate lymphocytes that destroy virus-infected cells and tumour cells without prior sensitisation.
• Dendritic cells: antigen-presenting cells that bridge innate and adaptive responses by processing pathogen fragments and presenting them to T lymphocytes.
• Complement system: a cascade of plasma proteins that opsonise pathogens, recruit immune cells, and can directly lyse bacterial membranes.
• Pattern recognition receptors (PRRs): molecular sensors such as Toll-like receptors (TLRs) that detect conserved pathogen-associated molecular patterns (PAMPs).

Adaptive immune system

The adaptive immune system, present only in jawed vertebrates, mounts highly specific responses and retains immunological memory, enabling faster and stronger reactions upon subsequent encounters with the same antigen.

T lymphocytes

T cells mature in the thymus and are subdivided into functional populations:

• Helper T cells (CD4+): coordinate immune responses by secreting cytokines that activate B cells, cytotoxic T cells, and macrophages.
• Cytotoxic T cells (CD8+): directly kill infected or cancerous host cells displaying foreign peptides on MHC class I molecules.
• Regulatory T cells (Tregs): suppress excessive immune activation, maintaining self-tolerance and preventing autoimmunity.

B lymphocytes and antibodies

B cells mature primarily in the bone marrow and, upon activation, differentiate into plasma cells that secrete antibodies (immunoglobulins). Antibodies bind specifically to antigens, neutralising pathogens and tagging them for destruction. Long-lived memory B cells persist after an infection resolves, enabling rapid antibody production on re-exposure.

Immune memory and vaccination

Immunological memory is the cellular basis of vaccination. When an individual is first exposed to an antigen — either through natural infection or immunisation — a subset of activated T and B cells differentiates into long-lived memory cells. On re-exposure, these memory cells expand rapidly, producing a secondary immune response that is faster, larger, and more effective than the primary response. This principle underpins the protective efficacy of vaccines against diseases such as measles, polio, and influenza.

Immune disorders

Dysfunction of the immune system manifests across a spectrum of conditions.

Immunodeficiency

Primary immunodeficiencies are inherited genetic disorders that impair specific components of the immune response, such as severe combined immunodeficiency (SCID). Secondary immunodeficiencies arise from external causes, including infection with HIV (which depletes CD4+ helper T cells, causing AIDS), malnutrition, or immunosuppressive therapy.

Autoimmune diseases

When central or peripheral tolerance mechanisms fail, self-reactive lymphocytes escape elimination and attack host tissues, resulting in autoimmune disease. Examples include rheumatoid arthritis (joints), systemic lupus erythematosus (multiple organs), and type 1 diabetes (pancreatic beta cells).

Hypersensitivity and allergy

Hypersensitivity reactions occur when immune responses to innocuous antigens cause tissue damage. Type I hypersensitivity (allergy and anaphylaxis) is mediated by IgE antibodies and mast cell degranulation. Types II, III, and IV involve antibody-mediated, immune-complex-mediated, and cell-mediated mechanisms, respectively.

Cancer immunology

The immune system surveys the body for neoplastic cells in a process called immune surveillance. Tumour cells can evade this surveillance by downregulating MHC molecules or expressing inhibitory signals. Modern immunotherapy approaches, such as immune checkpoint inhibitors and CAR-T cell therapy, aim to restore or enhance immune recognition of tumours.

Regulation and self-tolerance

To prevent destruction of the body's own tissues, the immune system distinguishes self from non-self through a process called self-tolerance. Central tolerance occurs in the thymus (for T cells) and bone marrow (for B cells), where self-reactive lymphocytes are eliminated by clonal deletion. Peripheral tolerance mechanisms — including anergy, regulatory T cells, and inhibitory receptors — further suppress auto-reactive cells that escape central deletion.

The major histocompatibility complex (MHC), encoded in humans by the HLA gene region, is central to antigen presentation. MHC molecules present peptide fragments of intracellular or extracellular proteins to T cells, enabling discrimination between infected and healthy cells.

Frequently asked questions

Is the immune system the same as immunity?

The immune system refers to the anatomical and cellular apparatus, whereas immunity describes the functional state of protection it confers. A fully functional immune system produces immunity, but the terms are not interchangeable.

Is innate immunity faster than adaptive immunity?

Yes. Innate immune responses are activated within minutes to hours of pathogen encounter, while adaptive immune responses typically take several days to a week or more to develop because they require clonal selection and expansion of specific lymphocytes.

Is the immune system capable of fighting cancer?

The immune system does identify and eliminate many nascent tumour cells through immune surveillance. However, cancers can evolve mechanisms to evade immune detection, which is why research into cancer immunotherapy continues to be an active and rapidly advancing field.

Is it possible to boost the immune system through diet or lifestyle?

Maintaining adequate nutrition, sleep, and physical activity supports normal immune function, but there is limited rigorous clinical evidence that any specific supplement or intervention boosts immune responses beyond normal homeostatic levels in healthy individuals. Deficiencies in micronutrients such as vitamins C, D, and zinc are, however, associated with impaired immunity.

Is immune system function the same across all ages?

No. Immune function changes across the lifespan. Neonates rely heavily on maternal antibodies; elderly individuals typically exhibit immunosenescence, a gradual decline in immune responsiveness associated with increased susceptibility to infection and reduced vaccine efficacy.