Why is lead dangerous?
Lead acts as a poison in a variety ways relative to what tissue it is exposed to. For example, lead is able to pass through the blood-brain barrier where it can act act as Calcium homolog to interfere with neurotransmitter release. In the blood, Lead inhibits porphobilogen synthase, an enzyme required for heme synthesis.

Lead's toxicity mechanisms include: 


Its ability bind sulfhydryl groups, common to many enzymes 
Its ability displace metals in enzyme complexes
Its ability to act as a Calcium homolog




Lead's valence electron configuration is $\require{mhchem} \ce{[Xe] 4f^14 5d^10 6s^2 6p^2}$; which means it has two electrons in its valence shell. This pair of valence electrons contribute's to Lead's ability to act as a Calcium analog and to Lead's tendency to form compounds in which it has an oxidation state of +2. Many such compounds dissolve in lightly acidic solutions. This has created lead exposure hazards when acidic tap water was pumped through lead pipes and when fruit juices have been served from ceramic vessels with lead-containing glazes. 

Lead Poisoning

Lead