The second line of defense is made up of white blood cells . Many white blood cells roam around the body, ready to attack invaders as soon as they get inside the body. Sometimes, however, the pathogens may reproduce too fast, spread too far or just outnumber the regular white blood cells. In such cases, the body's defense will be in the hands of the T cells and B cells. The third line of defense is made up of T cells .
T cells are, in fact, special kinds of white blood cells.
T cells strike at and destroy diseased cells - i.e. cells that they recognize as being infected by a pathogen. They can recognize these cells because infected cells display foreign proteins called antigens. The Intelligence Officers B cells , also a special kind of white blood cell, are not themselves a defensive force as such. They do not actively neutralize or destroy pathogens. But they serve a crucial function in identifying particular pathogens and releasing the appropriate antibodies.