The immunoglobulin that can cross the placenta is ?

HPSSC Staff Nurse - 2021

Applied Physiology

Easy

Appeared in: HPSSC Staff Nurse - 2021

Explanation

Immunoglobulin G (IgG) is the most abundant antibody in the blood and is the only class that can be transported across the placenta from mother to fetus.
This ability is due to its relatively small molecular weight and the presence of specific receptors on placental cells that bind to the Fc (fragment, crystallizable) region of the IgG molecule.
This process, known as transplacental transport, provides the fetus and newborn with passive immunity against infections that the mother has been exposed to.
Maternal IgG protects the infant for the first few months of life until its own immune system becomes fully functional.

Option A: IgA is the primary secretory antibody found in mucous membranes, saliva, tears, and breast milk. It provides local immunity at mucosal surfaces but does not cross the placental barrier.
Option B: IgB is not a recognized class of human immunoglobulin. The five major classes are IgG, IgA, IgM, IgD, and IgE.
Option D: IgE is primarily involved in mediating immediate hypersensitivity reactions (allergies) and in the defense against helminthic (parasitic worm) infections. It does not cross the placenta.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Placental transfer of immunoglobulins as background academic context rather than a clinical decision trigger.
The transfer of maternal IgG is a cornerstone of neonatal immunity, protecting infants from diseases like measles, tetanus, and diphtheria before they can be vaccinated.
This passive immunity wanes over the first 6 to 12 months of life, which is why the infant vaccination schedule is timed to begin building the baby's own active immunity.
What if? If a mother has an autoimmune disease (e.g., myasthenia gravis, lupus), the pathogenic autoantibodies, which are typically IgG, can also cross the placenta. This can lead to a temporary (transient) form of the disease in the newborn, which resolves as the maternal antibodies are cleared from the infant's circulation.

How to Approach the Question

First, identify the key terms in the question: 'immunoglobulin' and 'cross the placenta'. This is a direct factual recall question from immunology.
Recall the five major classes of immunoglobulins: IgG, IgA, IgM, IgE, and IgD.
Think about the specific function of each class related to maternal-fetal transfer. A helpful mnemonic is 'IgG Goes across the placenta'.
Evaluate the options based on this knowledge. IgA is secretory (found in milk), IgE is for allergies, and IgM is too large.
Recognize that 'IgB' is not a valid class of immunoglobulin, making it an incorrect choice.
Conclude that IgG is the only option that fits the criteria described in the question.

Concept Tested & Keywords

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