Hemoglobin?

Ok, iam a blood transfusions volunteer. In Indonesia, before someone standard to donate their blood hemoglobin test is required, i know that famine of Hb is bad, but why? how harmful? any solution to increase it?
Im really need those info, could someone answer my quiz or give me links, thankfulness so muchHemoglobin?
A lack of hemoglobin mechanism the person is iron not up to standard or anemic, so donating blood would be dangerous
hgb carry oxygen to cells.....if hgb low,way cells find low oxygen resulting in thinness,fainting,brain function problems,etc
.also,muscles call for oxygen to work,ie,YOUR HEART....
Lack of hemoglobin means the party is anemic and does not have iron cell (I think that's what they phone up it). So the person is unfit to donate blood while he himself is within requirement of blood! lol