The immune system is composed of an innate (non-specific) and an adaptive (specific) response. Innate immunity is constitutively present and is mobilized immediately following infection. Innate immunity is termed non-specific because the protective response is the same regardless of the initiating infection. This is in contrast to the adaptive immune system which is slower, responds specifically, and generates an immunological memory.
The primary component of innate immunity is inflammation. Injured cells release cytokines and other pro-inflammatory factors (i.e. bradykinin, histamine, leukotrienes, prostaglandins, serotonin) to contain the spread of infection and promote healing. These pro-inflammatory mediators induce vasodilatation and attract phagocytes. Neutrophils subsequently further the response by attracting leukocytes and lymphocytes. Activation of the Complement cascade enhances the innate response. An important consequence of complement cascade activation is opsonization. Opsonization of pathogenic antigens tags invasive microorganisms for ingestion and destruction by phagocytes. The innate response involves a wide number of cell types but is particularly dependent on basophils and mast cells (inflammation) and neutrophils and macrophages (phagocytosis). Another important function of the innate immune system is to stimulate the adaptive immune response via antigen presentation.