Adaptive immunity is triggered when a pathogen evades the innate immune system for long enough to generate a threshold level of an antigen. An antigen is any molecule that induces an immune response, such as a toxin or molecular component of a pathogen cell membrane, and is unique to each species of pathogen. A typical adaptive immune response includes several steps:
- The antigen for the pathogen is taken up by an antigen-presenting cell (APC), such as a dendritic cell or macrophage, through phagocytosis.
- The APC travels to a part of the body that contains immature T and B cells, such as a lymph node.
- The antigen is processed by the APC and bound to MHC class II receptors and MHC class I receptors on the cell membrane of the APC.
- The antigen is presented to immature helper T cells and cytotoxic T cells through binding the MHC II (helper T) or MHC I (cytotoxic T) to T-cell receptors.
- These T lymphocytes mature and proliferate. Helper T cells activate B cells, which proliferate and produce antibodies specific to the antigen, while cytotoxic T cells destroy pathogens that bear the antigen that was presented to them by the APCs.
- Memory B and T cells are formed after the infection ends.
Antigen Presentation: Antigen presentation stimulates T cells to become either “cytotoxic” CD8+ cells or “helper” CD4+ cells. Cytotoxic cells directly attack cells carrying certain foreign or abnormal molecules on their surfaces. Helper T cells, or Th cells, coordinate immune responses by communicating with other cells. In most cases, T cells only recognize an antigen if it is carried on the surface of a cell by one of the body’s own MHC, or major histocompatibility complex, molecules.