<P> A B cell identifies pathogens when antibodies on its surface bind to a specific foreign antigen . This antigen / antibody complex is taken up by the B cell and processed by proteolysis into peptides . The B cell then displays these antigenic peptides on its surface MHC class II molecules . This combination of MHC and antigen attracts a matching helper T cell, which releases lymphokines and activates the B cell . As the activated B cell then begins to divide, its offspring (plasma cells) secrete millions of copies of the antibody that recognizes this antigen . These antibodies circulate in blood plasma and lymph, bind to pathogens expressing the antigen and mark them for destruction by complement activation or for uptake and destruction by phagocytes . Antibodies can also neutralize challenges directly, by binding to bacterial toxins or by interfering with the receptors that viruses and bacteria use to infect cells . </P> <P> Evolution of the adaptive immune system occurred in an ancestor of the jawed vertebrates . Many of the classical molecules of the adaptive immune system (e.g., immunoglobulins and T - cell receptors) exist only in jawed vertebrates . However, a distinct lymphocyte - derived molecule has been discovered in primitive jawless vertebrates, such as the lamprey and hagfish . These animals possess a large array of molecules called Variable lymphocyte receptors (VLRs) that, like the antigen receptors of jawed vertebrates, are produced from only a small number (one or two) of genes . These molecules are believed to bind pathogenic antigens in a similar way to antibodies, and with the same degree of specificity . </P> <P> When B cells and T cells are activated and begin to replicate, some of their offspring become long - lived memory cells . Throughout the lifetime of an animal, these memory cells remember each specific pathogen encountered and can mount a strong response if the pathogen is detected again . This is "adaptive" because it occurs during the lifetime of an individual as an adaptation to infection with that pathogen and prepares the immune system for future challenges . Immunological memory can be in the form of either passive short - term memory or active long - term memory . </P> <P> Newborn infants have no prior exposure to microbes and are particularly vulnerable to infection . Several layers of passive protection are provided by the mother . During pregnancy, a particular type of antibody, called IgG, is transported from mother to baby directly through the placenta, so human babies have high levels of antibodies even at birth, with the same range of antigen specificities as their mother . Breast milk or colostrum also contains antibodies that are transferred to the gut of the infant and protect against bacterial infections until the newborn can synthesize its own antibodies . This is passive immunity because the fetus does not actually make any memory cells or antibodies--it only borrows them . This passive immunity is usually short - term, lasting from a few days up to several months . In medicine, protective passive immunity can also be transferred artificially from one individual to another via antibody - rich serum . </P>

Briefly describe the role of the nervous system in regulating the immune response