Re: how and why do hormones act in opposing pairs?

Hormones act to keep the body in balance (homeostasis) and to enable it to respond to emergencies. It is much more complicated than sets of opposing pairs, although it is possible to think of some examples that fit that idea.

I suppose you are thinking of the two steroid hormones testosterone and estrogen, which lead to male or female development. In the sense that male and female are "opposites", your observation is valid. However, estrogen is present in males and is important in males for brain development, cardiac health and strong bones. Testosterone is present in females, and is necessary for building muscles and for other functions.

Hormones such as estrogen and testosterone are steroid hormones. They are manufactured in various tissues such as the testis, ovaries, brain, and adrenal glands using cholesterol as a starting material. There are many different steroid hormones, each with numerous effects on development and homeostasis. Steroid hormones control cells by engaging an intracellular receptor (a protein) which in turn can turn on (or off) the genes needed for a particular response. I suggest a college level Endocrinology textbook for more infomation on this topic.

Steroid hormones are far from the only type of hormone used in the body. Proteins, peptides, and other small molecules are also used as hormones and play crucial roles in development, response to injury and homeostasis.

Some good references for you to follow up on: http://www.hormone.org/ resources/factsheets.html http://www.yale.edu/ynhti/ curriculum/units/1988/5/88.05.04.x.html