Innate immunity is not always sufficient to provide an organism's protection. At this point an adaptive immune response is activated. Adaptive immune response is an inflammatory process that obligatorily involves lymphocytes. Adaptive immune response is more active and aggressive as compared to innate inflammation. Yet the latter is necessary for adaptive immune response activation.
Main "objectives" of adaptive immune response are the following: lymphocytic recognition of both pathogen and pathogen-damaged cells or tissues of organism; destruction of the pathogen and damaged cells and clearance of the destruction products from the organism.
Features of adaptive immune response
• Lymphocyte clones. Lymphocytes are the only cell type in which differentiation involves somatic DNA recombination of the genes encoding antigen receptors. This generates an unprecedented diversity of lymphocyte clones each with a unique antigen-specific receptor, namely: 1018 variants for T-lymphocytes and 1016 variants for B-lymphocytes.
• Binding of an Ag-recognizing lymphocyte Rc to its ligand is necessary but insufficient to initiate an immune response.
• Costimulatory molecules. To initiate an immune response a T-lymphocyte requires two signaling events. Antigen-specific signal comes from TCR binding peptide-MHC class I or II complex presented by an APC (antigen-presenting cell). A second activation signal is provided by the interaction of costimulatory molecules expressed by T-cells and APCs.
• Cells of innate inflammation do not possess a wide Rc variety. Their Rc are invariant, conservative, and encoded by germline genes. Nevertheless these Rc are the first to selectively bind microbial products which are atypical for the macroorganism. Therefore Rc of innate inflammation cells are the "carriers of evolution memory". They are on the front line of defense to distinguish "foreign" from "self" and to inform lymphocytes about the invasion of "foreign" into the internal environment.