**2.2 Adaptive immune response**

The adaptive immune response has the ability to recognize specific antigens and to remember those antigens in case of a subsequent exposure: this immunological memory allows a very rapid response, as the particular antigen has been already previously encountered and recognized. T and B lymphocytes, mediators of this response, undergo clonal expansion when they encounter the specific antigen they are programmed to recognize. At that moment, lymphocytes experience a real metabolic switch, increasing their metabolic needs for glucose and aminoacids, and passing from the normal oxidative phosphorylation typical of naive cells to aerobic glycolysis, in which pyruvate produced by glycolysis is reduced to lactic acid, with the simultaneous generation of NAD+ molecules, which promote the continuous production of 2 ATP molecules for each metabolized glucose molecule. This process, which occurs in the presence of oxygen, is less efficient than oxidative phosphorylation, but much faster, and thus able to meet the high ATP demand required to rapidly increase the biosynthesis of lipids, proteins, and nucleic acids of activated lymphocytes [15]. T lymphocytes exclusively recognize antigenic peptides exposed on the membrane of APCs via the major histocompatibility complex (MHC), whereas B lymphocytes recognize soluble, circulating antigens. Lymphocytes are present in an immature form in the primary lymphoid organs (bone marrow and thymus), where they differentiate into mature lymphocytes through a particular process of nonhomologous genetic recombination in the genes coding for antigen receptors (antibodies for B lymphocytes and T-cell receptors (TCR) for T lymphocytes). This somatic gene rearrangement accounts for the vast heterogeneity of lymphocytes, allows each individual to have a large and unique immunological repertoire, able to recognize a very large number of molecular configurations present in foreign agents, and thus counteract the majority of infections encountered during life [6].
