FIGURE 23-1 The three layers of vertebrate immune defenses. Left: Mechanical defenses consist of epithelia and skin. Chemical defenses include the low pH of the gastric environment and antibacterial enzymes in tears. These barriers provide continuous protection against invaders. Pathogens must physically breach these defenses (step 1) to infect the host. Middle: Pathogens that have breached the mechanical and chemical defenses (step 2) are handled by cells and molecules of the innate immune system (blue), which includes phagocytic cells (neutrophils, dendritic cells, macrophages), natural killer (NK) cells, complement proteins, and certain interleukins (IL-1, IL-6). Innate defenses are activated within minutes to hours of infection. Right: Pathogens that are not cleared by the innate immune system are dealt with by the adaptive immune system (step 3), in particular B and T lymphocytes. Full activation of adaptive immunity requires days. The products of an innate response may potentiate an ensuing adaptive response (step 4). Likewise, the products of an adaptive immune response, including antibodies (Y-shaped icons), may enhance innate immunity (step 5). Several cell types and secreted products straddle the fence between the innate and adaptive immune systems and serve to connect these two layers of host defense.