There are billions of different kinds of T cells, each of them born to recognize one particular antigen -- that's the little piece of the pathogen displayed by a phagocyte after it gobbles an invading pathogen. The recognition of one special antigen by a single T cell is one of the most astounding aspects of the adaptive immune response. The human immune system can recognize a staggering 100 billion different antigens displayed on the surface of the phagocytes. Based on that recognition, the adaptive immune system is able to customize its terminator strategy to a particular antigen. Since each T cell is born to recognize one antigen, this provides the body with a huge repertoire of cells predestined to recognize any of the germs likely to invade your body. But out of all these microorganisms, and all the T cells designed to respond to those microorganisms, how does the one special T cell get activated and copied when the microbe it targets enters the body?
It looks for two kinds of signals from the phagocyte that has chopped up the microbe. The first signal is the piece of antigen displayed on the phagocyte's surface. The second signal is the all-important danger flag triggered by the pattern-recognition receptors (PRRs) of the cells of the innate immune system. This second signal is essential because it tells the T cells to multiply and ramp up their fighting forces. Without this danger signal, there'd be no serious immune attack.
When an antigen is presented to its matching T cell for the first time, the T cell locks on to an antigen receptor on the phagocyte's surface. This is like a key fitting a customized lock. Binding with the antigen causes the T cell to start making clones -- identical copies of itself that are able to recognize that specific antigen. Where once there was one T cell, over the course of three to five days, a small army of T cell clones comes into being. This army organizes itself into squadrons with assigned tasks. Some are designated to kill infected cells; some activate other lymphocytes to multiply; others, known as memory T cells, remember the antigen and wait quietly for a return visit at a later date.
Because these T cells are so specific, once a T cell recognizes its antigen, it can launch an attack that is tailored to the way that pathogen operates. It anticipates, for example, whether the pathogen will operate inside or outside a cell, and can organize its attack based on that information.