"Humans and other warm-blooded animals have an additional defensive capability called adaptive immunity, which differentiates specifically between pathogens. Adaptive immunity is due to the actions of two types of specialized leukocytes, known as T cells and B cells. T cells and B cells have recognition methods that distinguish between different pathogens, such as different species of bacteria, and they adapt during their first encounter with a particular pathogen. The second time they meet it in the body, the adaptive response begins earlier, lasts longer and is more effective than it was on the first occasion. This response is due to the production of long-lived memory cells that circulate in the body after the primary adaptive immune response subsides. These memory cells are specifically programmed to recognize the same pathogens that triggered the primary response if they ever get into the body again. Every type of pathogen has at least one, and often many more, unique molecules known as antigens in its structure. In addition to the PAMPs shared by many different pathogens, each type of pathogen also has its own distinctive antigens. Each individual T cell and B cell is programmed to recognize just one specific antigen, so each can usually recognize only one type of pathogen, or at most two or three closely related pathogens that have very similar antigens. Recognition of an antigen by these adaptive leukocytes triggers an immune response against only those pathogens with that antigen in their structure. Antibodies are very large proteins that contribute to adaptive immunity. Their structure includes at least two binding sites for antigens. B cells produce antibodies and also use them as their antigen receptors. The B cells carry antibodies embedded by the tail in their outer cell membrane, with the binding sites facing outwards. This enables the B cell to bind to antigens that fit the binding sites in the antibodies it carries on its surface. This binding event is essential, but not sufficient on its own, to activate B cells into making many more antibody molecules that recognize the same antigen. These antibodies are released by the B cells and circulate in the bloodstream, tissue fluids and the lymphatic system. Antibodies are also abundant in the mucus membranes lining the respiratory system, gut and reproductive system. When antibodies bind to a pathogen, they label it for destruction by leukocytes with the innate ability to phagocytose it, or by cytotoxic leukocytes and defensive proteins. There are two types of T cells with different roles in adaptive immunity. Cytotoxic T cells release destructive chemicals onto their target’s outer surface in much the same way as cytotoxic leukocytes do in an innate immune response."