Q11.A class of Immunoglobulin present in the serum which neutralizes toxins, activates complements and crosses the placenta to protect the fetus and neonate is:
(A) IgD
(B) IgE
(C) IgG
(D) IgM
IgG (C) is the immunoglobulin present in serum that neutralizes toxins, activates complements, and crosses the placenta to protect the fetus and neonate.
It constitutes 75-80% of serum antibodies, making it the most abundant class with these specific functions.
Option Breakdown
IgD (A): Primarily functions as the B-cell receptor on naive B lymphocytes, involved in B-cell activation and differentiation. Low serum levels (~0.25 mg/mL); does not activate complement efficiently, cannot cross placenta, no toxin neutralization role.
IgE (B): Mediates type I hypersensitivity reactions, binds mast cells/basophils for parasite defense and allergy responses. Very low serum concentration (~0.05 μg/mL); does not activate complement, no placental transfer, minimal toxin neutralization.
IgG (C): Dominant serum immunoglobulin (10-16 mg/mL). Activates classical complement pathway via Fc region, neutralizes bacterial toxins/viruses through Fab binding, only Ig class crossing placenta via FcRn receptor for neonatal passive immunity.
IgM (D): First responder in primary immune response, pentameric structure excellent for complement activation and agglutination. High molecular weight prevents placental crossing; serum levels ~1.5 mg/mL but short half-life (~5 days).
Introduction to Immunoglobulin Functions
IgG immunoglobulin serum neutralizes toxins crosses placenta defines the principal antibody providing long-term humoral immunity through secondary responses. As the only Ig crossing human placenta, maternal IgG protects neonates for first 6 months. Critical knowledge for NEET-PG, USMLE, CSIR-NET life sciences immunology sections.
Functional Characteristics of Antibody Classes
Each immunoglobulin class shows specialized roles:
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Complement activation: IgG (classical pathway), IgM (potent activator due to pentameric structure).
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Placental transfer: Exclusive to IgG via neonatal Fc receptor (FcRn) binding in syncytiotrophoblast.
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Toxin neutralization: IgG Fab domains bind exotoxins (tetanus, diphtheria), preventing cellular toxicity.
