There is only one Ca gene, but there are about 50 Va and 50 Ja genes. As a T cell develops, it randomly chooses one of the 50 Va genes and moves it next to one of the 50 Ja genes. However, when a B or T cell encounters an invader that can bind to its receptor, the cell divides many times and so creates lots of daughter cells. Each bears receptors that are the same as, or very similar to, those of the parent. T cells many more that can react with the invader.
Effector cells act to get rid of the invader.
The memory cells survive to protect their host against further infections by the same invader. B cells bearing antibodies and T cells bearing ab or gd receptors recognize the appearance of an invader in the body in different ways. B cell antibodies bind to the invading particle, such as a bacterium, in the form in which it enters the body. The ab receptor-bearing T cells do not bind the invader directly. Instead, they bind to peptide fragments made from the invader's proteins.