The adaptive immune response is mediated by immune cells known as lymphocytes. These are B and T cells. B cells secrete antibodies, highly specific protein molecules that bind to a specific pathogen. These antibodies bind specific parts of pathogens known as antigens - either presented extracellularly on infected cells or free-floating in the body. Antibody binding attracts mechanisms that will then attack and destroy the infected cell or pathogen. Some of these B cells become memory cells, which help the body “remember” the disease and prevent re-infection. T cells can either be helper T cells or cytotoxic T cells, and bind pathogens via the T-cell receptor (TCR), which senses specific protein sequences. Helper T cells activate B cells, attract macrophages, and secrete cytokines. Cytotoxic T cells create pores in infected cells through which they introduce chemicals that trigger apoptosis, thus actively killing the cell