What is the complement system is a barrier of the immune system.

The compliment system is a biological barrier of the immune system.

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.

the level of progesterone is low at the pre-ovulatory phase of the cycle but it gradually increases and the levels are high at the luteal phase. if pregnancy occurs, the level of progesterone will be high throughout the pregnancy.

if egg is fertilized and the mother gets pregnant, progesterone starts preparing the uterus for the implantation. it starts building up the lining of uterus and make it hostile of the period of pregnancy.