Adaptive immunity provides vertebrates with a

Position 1: It is precisely this immunological memory that vaccines exploit, training the immune system without exposing the host to the risks of active disease.

Position 2: This specificity arises from the somatic recombination of gene segments in developing lymphocytes, generating an enormous diversity of antigen receptors.

Position 3: After the infection is cleared, a subset of these cells persists as long-lived memory cells that enable a faster, stronger response upon re-exposure.

Position 4: Adaptive immunity provides vertebrates with a highly specific defense mechanism capable of recognizing and remembering an almost limitless variety of pathogens.

Position 5: When a receptor binds its complementary antigen, the corresponding lymphocyte undergoes clonal expansion, producing large numbers of effector cells.