This occurs predominately during the third trimester of pregnancy, and thus is often reduced in babies born prematurely. Immunoglobulin G (IgG) is the only antibody isotype that can pass through the human placenta, and is the most common antibody of the five types of antibodies found in the body.

What antibodies can cross?

Placental transfer of maternal IgG antibodies to the fetus is an important mechanism that provides protection to the infant while his/her humoral response is inefficient. IgG is the only antibody class that significantly crosses the human placenta.

Which IgG can cross placenta?

It was found that all subclasses of IgG (IgG1–IgG4) cross the human placenta (12). IgG1 is the most efficiently transported subclass and IgG2 the least (13).