Figure 1.

Innate and adaptive immunity. A simplified schematic of the two branches of the immune response. Following injury or infection, pathogens (bacteria, virus or foreign protein) infiltrate tissue. The innate response provides immediate defense against infection (1–5): 1. Neutrophils engulf the pathogen and destroy it by releasing antimicrobial toxins. 2. Macrophages can directly phagocytose pathogens, leading to production of cytokines and recruitment of more cells from the blood. 3. Infected cells displaying low levels of MHCI on their surface are directly detected by natural killer (NK) cells, which release lytic enzymes causing the infected cell to die via apoptosis. 4. Bacteria can also be recognized by the complement system, resulting in their lysis. 5. Macrophages and dendritic cells can become antigen presenting cells (APCs) by taking up peripheral antigens and migrating to lymph nodes to present antigen on their surface to naïve B- and T- cells. The adaptive response confers the ability to recognize and remember specific pathogens to generate immunity (6–11): 6. APC interaction with B- and T- cells in the lymph nodes leads to B- and T-cell activation and migration to the periphery where they mediate adaptive immunity. 7. Once activated, the T-cell undergoes a process of clonal expansion in which it divides rapidly to produce multiple identical effector cells. Activated T-cells then travel to the periphery in search of infected cells displaying cognate antigen/MHCI complex. 8. Peripheral APCs induce helper T cells to release cytokines and recruit cytotoxic T cells (CTL). 9. Activated antigen-specific B cells receiving signals from helper T-cells differentiate into plasma cells and secrete antibodies. 10. Antibodies bind to target antigens forming immune complexes which can then activate complement or be taken up by macrophages through Fc receptors 11. Formation of cytotoxic T-cell synapses causes lysis of the infected cell.