This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Immunity, Subject name: Pathology
Which Antibody crosses placenta? 
Select and explain. 
A. IgG1, B. IgG4, C. IgA, D. IgD
A: IgG1
Answer: A: IgG1 
Explanation: Ans. (a) IgG1(Ref: Maternal, Fetal, & Neonatal Physiology: A Clinical Perspective; pg 484)All four IgG subclasses cross, although the IgG1 and IgG3 subclasses are predominate.IgG1 crosses earliest in pregnancy & is the primary immunoglobulin transferred before 28 weeks.IgG3 crosses later & does not reach maternal levels until after 32-33 weeks