Overview
Adaptive immunity is the antigen-specific, memory-forming arm of the immune response, carried out by B and T lymphocytes after they meet their antigen in the peripheral lymphoid organs. This topic follows the response from activation to effector cells, antibodies and memory, and ends with what happens to the antigen.
Humoral and cellular adaptive immunity. Humoral immunity is mediated by antibodies secreted by plasma cells (derived from B cells, usually with Th2 help); it acts mainly against extracellular microbes and toxins. Cellular immunity is mediated by T cells: helper T cells (Th1) activate macrophages and cytotoxic T cells, and cytotoxic T cells kill infected cells. Both arms need antigen-presenting cells and cytokines, and both leave memory cells.
- B and T lymphocytes complete their development and become immunocompetent in the central lymphoid organs (bone marrow for B cells and thymus for T cells).
- These immunocompetent cells then migrate to peripheral lymphoid organs where they participate in immune responses upon encountering antigens.
- These organs are the primary sites for initiation and regulation of immune responses.
- Lymphocytes in these organs become activated upon recognizing antigens.
- Activated lymphocytes undergo clonal proliferation and antigen-specific differentiation into effector cells (that eliminate pathogens) and memory cells (that persist for future immune responses).
- This process is antigen-dependent, unlike their earlier maturation which is antigen-independent.
- Effective immune responses depend on interactions between B cells, T cells, and antigen-presenting cells, all coordinated by signaling molecules called cytokines.