Q.3 Which of the following antibodies can cross the placenta?
1. IgA
2. IgG
3. IgM
4. IgE
Which Antibody Crosses the Placenta? IgA, IgG, IgM, or IgE Explained
Only IgG among the listed antibodies can cross the placenta to provide passive immunity to the fetus. This multiple-choice question frequently appears in biology and medical entrance exams like NEET.
Correct Answer
2. IgG.
IgG is the sole antibody class that transfers from maternal to fetal circulation via the neonatal Fc receptor (FcRn) on placental syncytiotrophoblast cells.
Option Breakdown
IgA
IgA primarily protects mucosal surfaces in secretions like saliva, tears, and breast milk but does not cross the placenta due to its structure.
IgG
IgG, the most abundant serum antibody (70-80%), crosses the placenta actively, mainly in the third trimester, protecting newborns against infections.
IgM
IgM, the largest antibody and first responder to infections, cannot cross the placenta because of its pentameric size and structure.
IgE
IgE mediates allergic reactions and parasite defense by binding mast cells but lacks the ability to cross the placental barrier.
| Antibody | Placenta Crossing | Primary Role | Structure Note |
|---|---|---|---|
| IgA | No | Mucosal immunity | Dimeric/secretory |
| IgG | Yes | Fetal passive immunity | Monomeric |
| IgM | No | Primary infection response | Pentameric |
| IgE | No | Allergies/parasites | Monomeric |
