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Difficulty: Very hardHuman Physiology, Health, and Diseases

Which of the following statements regarding human immunoglobulin functions, immune cell receptor interactions, and pathogen neutralization mechanisms are correct?

  1. Secretory Immunoglobulin A (IgA) exists predominantly as a dimeric molecule joined by a Joining (J) chain and a secretory component, providing primary mucosal immunity against respiratory and enteric pathogens.Answer
  2. Immunoglobulin E (IgE) binds via its Fc region to high-affinity FcεRI receptors on mast cells and basophils, triggering mediator release in type I hypersensitivity reactions and anti-helminthic immunity.Answer
  3. C
    Immunoglobulin M (IgM) circulates as a monomer in human serum that exclusively mediates long-term secondary memory immune responses against intracellular bacterial pathogens.
  4. Immunoglobulin G (IgG) is the only human antibody class capable of crossing the placental barrier, a process mediated by active transport via neonatal Fc receptors (FcRn).Answer

Answer

The statements confirming that secretory IgA functions as a dimeric mucosal barrier, IgE binds FcεRI receptors on mast cells to mediate type I hypersensitivity and helminth defense, and IgG crosses the placenta via neonatal Fc receptors (FcRn) are correct.
The correct statements accurately detail human immunoglobulin physiology: Secretory IgA functions as a dimeric molecule linked by a J chain and secretory component for mucosal lining protection. IgE binds high-affinity FcεRI receptors on mast cells/basophils, mediating allergic degranulation and anti-helminth defense. IgG is actively transported across the placenta by neonatal Fc receptors (FcRn) to provide fetal passive immunity.

Step-by-Step Solution

1
Evaluate the structural and functional properties of Secretory IgA.
Secretory IgA is composed of two IgA monomers linked by a J chain and wrapped by a protective secretory component derived from polymeric immunoglobulin receptors. It protects mucosal membranes in the gastrointestinal, respiratory, and urogenital tracts.
Verifies that the statement on dimeric secretory IgA structure and mucosal pathogen protection is scientifically accurate.
2
Assess the receptor binding and physiological role of Immunoglobulin E (IgE).
IgE binds with high affinity to FcεRI receptors on mast cells and basophils. Antigen cross-linking induces immediate degranulation, releasing histamine and leukotrienes central to type I hypersensitivity and parasite immunity.
Confirms the correctness of the statement detailing IgE Fc receptor interaction and allergic/helminthic immune pathways.
3
Analyze the structural organization and temporal response of Immunoglobulin M (IgM).
Serum IgM is a pentameric complex held together by disulfide bonds and a J chain, specializing in agglutination and early complement activation during primary immune responses. It is not a monomer responsible for secondary long-term memory.
Identifies the incorrect statement mischaracterizing IgM monomeric state and secondary immune memory functions.
4
Examine the mechanism of placental antibody transfer in human pregnancy.
IgG is the sole antibody isotype that crosses the placental barrier into fetal circulation. This active transcytosis is mediated by placental neonatal Fc receptors (FcRn) binding the Fc portion of IgG.
Validates that maternal-fetal IgG placental transport via FcRn confers essential passive immunity.

Key Concept

Structure, isotype-specific effector functions, and receptor interactions of human immunoglobulins in mucosal defense, passive immunity, and hypersensitivity.
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Topics

Human Physiology, Health, and Diseases

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