The immune system is composed of two major subdivisions, the innate or non-specific immune system and the adaptive or specific immune system. Innate immunity that is constitutively present and is immediately mobilized upon infection acts as the first line of defense against invading organisms. While the adaptive immune system acts as a second line of defense and also affords protection against re-exposure to the same pathogen. Innate immunity is referred to as “non-specific” for the innate immune response is not antigen specific and reacts equally well to a variety of organisms. In contrast, the adaptive immune system responds specifically and generates immunological memory.
Like adaptive immunity, innate immunity also has humoral response and cell-mediated response. In addition, the innate immune system also has anatomical barriers to infection, including mechanical factors (skin and internal epithelial layers), chemical factors (i.e. lysozyme, phospholipase, defensins) and biological factors (secreted toxic substances). Once the anatomical barriers are breached, another innate defense mechanism comes into play, namely acute inflammation. Injured cells release cytokines and other pro-inflammatory factors (i.e. bradykinin, histamine, serotonin leukotrienes, prostaglandins) to control the spread of infection and promote healing. These chemical factors cause vasodilation of the blood vessels and recruit phagocytes to sites of infection. Activation of complement cascade enhances inflammatory responses, and causes lysis and opsonization of infectious organisms. Complement system, coagulation system, lysozymes, cytokines and other secreted substances constitute the humoral immunity system of innate immunity. The cell-mediated response involves a wide number of cell types, including phagocytes (neutrophiles, monocytes/macrophages, and dendritic cells), natural killer cells (NK cells), basophils, mast cells and eosinophils. Macrophages and dendritic cells are major antigen-presenting cells and serve as a link between the innate and adaptive immune systems.