The innate immune system is phylogenetically older than the acquired (specific/adaptive) immune system.Latter might take 3-5 days to be effective. Therefore,  there needs to be a system capable of controlling a pathogen during that time so it doesn’t damage the host.

Innate immune system is activated immediately and can rapidly control the replication of the infecting pathogens until the lymphocytes can deal with it. Innate immune system can interact with and control the adaptive immune responses.

The strategy of the IIS is to recognize a few highly conserved structures present in large groups of organisms. These structures are called pathogen-associated molecular patterns (PAMP), and the receptors of the IIS that recognize them are called pattern-recognition receptors PAMPs are produced only by microbial pathogens that may include bacterial lipopolysaccharides, peptidoglycan, mannas, and bacterial DNA/RNA.

Once the macrophages interact with PAMPs, they release cytokines (IL-1, TNFα, IL-8) that recruit and activate neutrophils and other leukocytes. Other cytokines that are released enhance microbial activities of phagocytes (interferon γ) and stimulate natural killer (IL-15) and Th1 cell mediated immune responses (IL-12, IL-18).

Pattern-recognition receptors that recognize PAMPs are  Mannan-binding lectin,  mannose receptor and  toll-like receptors-there are at least ten mammalian toll-like receptors. They are on cell surfaces and once stimulated, these receptors lead to activation of cell signaling pathaways that  results in induction of inflammatory and immune response genes with subsequent elaboration of cytokines.

Interaction with the acquired immune system

Pattern recognition receptors on antigen-presenting cells  of the innate immune system bind pathogens, which are endocytosed. The phagocytized pathogen can then be processed and presented to T cells in the context of MHC class II molecules.

Furthermore, binding of PAMPs to toll-like receptors signals the APC to upregulate CD80 (B7-1)/CD86 (B7-2) costimulatory molecules on the surface of the APC. Only when the APC presents both class II molecules with antigen to the T cell receptor and CD80 (B7-1) or CD86 (B7-2) to CD28 on the T cell can the T cell be activated resulting in stimulation of the acquired immune system.

Self antigens are not recognized by receptors of the innate immune system.