Adaptive immunity is the branch of the vertebrate immune system that develops highly specific responses to antigens and provides long-lasting protection through immunologic memory. Unlike innate defences that act broadly and immediately, adaptive responses are learned during the lifetime of an organism and improve with each encounter.
Principles and components of adaptive immunity
The adaptive immune system relies on lymphocytes with antigen receptors generated by somatic gene rearrangements, enabling recognition of a vast array of pathogens. B lymphocytes, upon activation by antigen and T helper cell signals, proliferate and differentiate into antibody-secreting plasma cells and memory B cells; their antibodies neutralize toxins, opsonize microbes and activate complement. T lymphocytes fall into several classes: helper T cells coordinate immune responses by producing cytokines; cytotoxic T cells directly kill infected or malignant cells presenting peptides on MHC class I molecules; and regulatory T cells limit excessive inflammation. During development, lymphocytes undergo positive and negative selection to ensure self tolerance. Key features of adaptive immunity include specificity for unique epitopes, diversity generated by V(D)J recombination, clonal expansion of antigen-specific cells, and memory that leads to faster and stronger secondary responses. These properties allow vertebrates to tailor responses to particular pathogens while preserving host tissues.
Responses and clinical significance
Adaptive immunity manifests as humoral and cell‑mediated responses. Vaccination exploits humoral memory by exposing individuals to antigens in a safe form to induce protective antibodies against diseases such as measles, tetanus and COVID‑19. Cell‑mediated immunity is critical for clearance of intracellular pathogens like viruses and Mycobacterium tuberculosis and for tumour surveillance. Disorders of adaptive immunity include immunodeficiencies, in which defective B or T cell responses lead to recurrent infections, and autoimmune diseases, where self‑reactive lymphocytes escape tolerance and attack host tissues. Therapeutically, monoclonal antibodies, checkpoint inhibitors and adoptive T cell therapies harness adaptive mechanisms to treat cancers and chronic infections. Understanding adaptive immunity also informs transplant rejection, allergy, and the design of new vaccines and immunotherapies.
Adaptive immunity provides organisms with flexible, antigen-specific defence and the ability to remember past exposures. By generating diverse receptors and expanding selected clones, the adaptive immune system can neutralize and eliminate a wide spectrum of pathogens while preserving host integrity.
Related Terms: Innate immunity, Antigen, B cell, T cell, Memory cell