- Adaptive immunity is a highly specific defense system that develops throughout our lifetime in response to exposure to pathogens and provides immunological memory. Unlike innate immunity, adaptive responses are tailored to specific threats and improve with each exposure.
- The two main branches of adaptive immunity are humoral immunity and cell-mediated immunity. Humoral immunity is mediated by B lymphocytes, which produce antibodies specific to particular pathogens or their products. These antibodies can neutralize toxins, mark pathogens for destruction, and activate other immune components. B cells can also develop into memory cells, enabling rapid responses to subsequent exposures of the same pathogen.
- Cell-mediated immunity is orchestrated by T lymphocytes, which recognize infected or abnormal cells through their T cell receptors. Helper T cells (CD4+) coordinate immune responses by secreting cytokines that direct other immune cells. Cytotoxic T cells (CD8+) directly kill infected or cancerous cells. Regulatory T cells maintain immune tolerance and prevent autoimmune responses by suppressing excessive immune reactions.
- Antigen presentation is crucial for adaptive immunity. Professional antigen-presenting cells, particularly dendritic cells, process and display pathogen fragments on their surface via MHC molecules. T cells recognize these presented antigens in conjunction with costimulatory signals, leading to their activation and proliferation. This process, known as clonal expansion, generates large numbers of antigen-specific T cells.
- Immunological memory, a defining feature of adaptive immunity, allows for faster and stronger responses to repeated pathogen exposures. Memory B and T cells persist long after initial infection and can rapidly mobilize upon reencountering their specific antigen. This principle underlies vaccination, where controlled exposure to pathogen components generates protective memory without causing disease.
