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
[{'content': 'Which Antibody crosses placenta? \nSelect and explain. \nA. IgG1, B. IgG4, C. IgA, D. IgD', 'role': 'user'}, {'content': 'Answer: A \nExplanation: 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', 'role': 'assistant'}]