Which antibody is transported across the placenta?

Correct Answer: C

Rationale: The correct answer is C: IgG. IgG is the only antibody that can be transported across the placenta from the mother to the fetus, providing passive immunity to the newborn. This transfer of IgG occurs through the neonatal Fc receptor (FcRn) present in the placenta. IgA is primarily found in mucosal secretions, IgM is too large to cross the placenta, and IgE is involved in allergic reactions and parasitic infections, not placental transfer. Therefore, IgG is the only antibody capable of crossing the placenta and providing immune protection to the fetus.

Correct Answer: C

Rationale: Isotype switching in B cells involves changing the constant region to produce different antibody isotypes, such as IgG, IgA, or IgE. This is important for diversifying the functions of antibodies without altering antigen specificity. It allows for different effector functions, such as opsonization, neutralization, or complement activation. This process does not eliminate antigen-specific B cells, as it maintains the immunological memory. Isotype switching does not directly affect antibody affinity for antigens, which is primarily determined by somatic hypermutation and affinity maturation. It is not limited to producing IgM only, as B cells can switch to other isotypes based on the cytokine microenvironment.

Correct Answer: A

Rationale: Step-by-step rationale: 1. Atopy is a genetic predisposition to produce IgE antibodies in response to allergens. 2. IgE antibodies play a crucial role in allergic reactions. 3. Individuals with atopy have an exaggerated immune response to allergens. 4. Excess production of IgA antibodies (B) and suppressed immune response to allergens (C) are not characteristic features of atopy. 5. Delayed hypersensitivity reactions (D) involve a different immune mechanism compared to the immediate IgE-mediated response in atopy.

Correct Answer: B

Rationale: The correct answer is B because sensitization in contact hypersensitivity requires T cell presentation of a hapten-modified antigen. This process activates T cells, leading to immune response. Antigen-antibody complex formation (A) is more related to immune responses in humoral immunity. Mast cell degranulation (C) is associated with allergic reactions, not contact hypersensitivity. Cytokine suppression (D) would hinder, not promote, the immune response needed for sensitization in contact hypersensitivity.

Correct Answer: B

Rationale: The correct answer is B: Pulmonary hemorrhage and glomerulonephritis. Goodpasture's syndrome is an autoimmune disease characterized by the presence of anti-glomerular basement membrane antibodies, leading to damage in the kidneys (glomerulonephritis) and lungs (pulmonary hemorrhage). Granuloma formation (A), contact dermatitis (C), and bronchoconstriction and vasodilation (D) are not characteristic features of Goodpasture's syndrome. Granulomas are more commonly seen in conditions like sarcoidosis, contact dermatitis is an inflammatory skin condition, and bronchoconstriction and vasodilation are features of allergic reactions or asthma, not typically seen in Goodpasture's syndrome.