Only certain organs called the target organs respond to the presence of a specific hormone because

Because   receptors which are sensitive  to the specific hormones are present only in the target organs

Explanation:

Hormones  receptors are specialized proteins  which are sensitive to   particular hormone stimulus. They are responsible for receiving  hormones  signals as inputs in the target organs, to produce cascade of events as response in the target organs . They   usually need small concentration of specific hormones  to bind with to produce  large effects in  corresponding cells functions and morphology.

They can be located in cell membrane where they are called peptide  hormone receptors; thus they receive  input signals from peptide hormones from the cells surfaces,and stimulated cascade of events  to bring about responses inside  the cells. Therefore are also called trans-membrane receptors.

However,some can be integrated within the cytoplasm, where they are called nuclear receptors. to initiate cascade of events in  the cells which affect the functions of the cells either for secretion or intake of a particular substances.

answer: - b cells

explanation: -

so, the b cells mediate their action by identifying the antigens present on the surface of the pathogens and then producing antibodies in return to mediate its effect.