Final answer:
The immune system develops a wide variety of lymphocyte receptors, and upon antigen binding, it undergoes clonal expansion, producing many effector cells specific to that antigen.
Step-by-step explanation:
The immune system can develop receptors for a specific antigen before that antigen enters the body. Lymphocytes make a wide variety of receptors, and when an antigen binds and activates one of these receptors, the cell divides, making many effector cells. This process is called clonal expansion. When an antigen enters the body, only those lymphocyte clones whose receptors are activated are stimulated to proliferate. This immune response involves a polyclonal response, in which multiple lymphocyte clones are stimulated and proliferate. By the end of this process, there will be a large number of specific lymphocytes available to fight the infection, each with the unique receptor that recognized the antigen.