**Question:** Immunoglobulin crossing placenta is

A. Albumin
B. Immunoglobulin A (IgA)
C. Immunoglobulin G (IgG)
D. Immunoglobulin M (IgM)

**Correct Answer:** Immunoglobulin G (IgG)

**Core Concept:** The placenta is a temporary organ formed from the embryonic disc and plays a crucial role in fetal development. It enables the exchange of essential substances between the mother and the fetus. Immunoglobulins are a class of proteins produced by the immune system, specifically by B-cells, to neutralize pathogens and antigens. There are five types of immunoglobulins: IgA, IgD, IgE, IgG, and IgM.

**Why the Correct Answer is Right:** Immunoglobulin G (IgG) is the most abundant immunoglobulin in the blood, accounting for approximately 75% of the total immunoglobulins. IgG plays a vital role in the immune response, particularly in providing passive immunity to the fetus. IgG is a polymeric immunoglobulin composed of two heavy and two light chains. It can cross the placenta via the Fc receptor (FcRn) present on the fetal side of the placenta.

**Why Each Wrong Option is Incorrect:**

A. Albumin: Albumin is a plasma protein responsible for transporting fatty acids, hormones, and other small molecules. It does not cross the placenta due to its large size and lack of specific receptors.

B. Immunoglobulin A (IgA): IgA is primarily involved in mucosal immunity and is not the primary immunoglobulin responsible for passive immunity. Moreover, its molecular weight is too large for passive transport across the placenta.

C. Immunoglobulin M (IgM): IgM is involved in the primary immune response and is not the main immunoglobulin responsible for passive immunity in the placenta. Its molecular weight is also too large for passive transport.

D. Immunoglobulin E (IgE): IgE is involved in allergic reactions and does not play a significant role in passive immunity or crossing the placenta. Its molecular weight is too large for passive transport.

**Clinical Pearls:**

1. The placenta plays a key role in the immune tolerance of the fetus, allowing essential nutrients and antibodies but preventing harmful substances from entering the fetal circulation.
2. The placental transport of immunoglobulins primarily occurs via the transcytosis process, where large immunoglobulins are internalized by the fetal syncytiotrophoblast cells and then transported through the placenta and released into the fetal circulation.
3. The placenta is a semipermeable barrier, allowing the passage of molecules based on size, charge, and structure. In the case of immunoglobulins, the Fc region of IgG allows passive transport through the placenta.
4. IgG is the main immunoglobulin responsible for passive immunity in the fetus, protecting the neonate from pathogens and toxins before the development of the neonatal immune system.

5. The fetal-maternal interface in the placenta is