The second phase of the body's immune response to a foreign substance involves of/by the body's defenses.

The answer is; activation

The second phase of defense is called the adaptive immune response. It involved the recruitment of white blood cells (leucocytes) (mainly the B- and T- cells). The innate defense system, which is the first response to an invading pathogen, involves the body’s physical barriers such as the skin and mucous membrane.

the sugar-phosphate backbone forms the structural framework of nucleic acids, including dna and rna. this backbone is composed of alternating sugar and phosphate groups, and defines directionality of the molecule.

during the process of dna replication, brdu can replace thymidine and incorporate into the newly synthesized dna, cell proliferation thus can be reflected in the extent of brdu incorporation.