Figure 1.

Simple overview of the immune system. The three layers of the immune system (physical and biochemical barriers; cells such as monocytes, granulocytes, lymphocytes, and B and T cells; and antibodies or immunoglobulins) work together to protect the body against pathogens, utilizing the innate and adaptive defense mechanisms. All three layers are involved in the innate and immune systems. * The innate immune system comprises anatomical and biochemical barriers and an unspecific cellular response mediated mainly by monocytes, neutrophils, natural killer cells and dendritic cells; these work together to fight off pathogens before they can start an active infection. ** The adaptive immune system involves an antigen-specific response mediated by T and B lymphocytes that is activated by exposure to pathogens; this works with the innate immune system to reduce the severity of infection. ^a The complement system can work with both the innate and adaptive immune systems; ^b i.e., immunity from serum antibodies produced by plasma cells; ^c i.e., an immune response that does not involve antibodies, but responds to any cells that display aberrant major histocompatibility complex (MHC) markers, such as cells invaded by pathogens.