Which pathway of the complement system involves properdin, Factor B, and Factor D?

Correct Answer: B

Rationale: The correct answer is B: Alternative pathway. This pathway involves properdin, Factor B, and Factor D. Properdin stabilizes the C3 convertase complex formed by Factor B and Factor D, leading to the amplification of the complement cascade. The other choices are incorrect because: A: Classical pathway is initiated by antigen-antibody complexes binding to C1q. C: Lectin pathway is activated by lectins binding to carbohydrates on pathogen surfaces. D: Terminal pathway involves the formation of the membrane attack complex.

Correct Answer: B

Rationale: The correct answer is B because clonal selection during lymphocyte activation leads to the formation of memory and effector cells. This process involves the proliferation and differentiation of the activated lymphocyte into these specialized cells, which play crucial roles in immune responses upon encountering the same antigen in the future. Cytokine suppression (choice A) is not the direct outcome of clonal selection but may be a regulatory mechanism during immune responses. Antigen elimination without cellular proliferation (choice C) is not accurate as clonal selection involves the expansion of specific lymphocyte clones. Destruction of the lymphocyte (choice D) is not the outcome of clonal selection, as activated lymphocytes undergo proliferation and differentiation rather than destruction.

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: B

Rationale: The correct answer is B: Differentiation into effector and memory cells. During clonal selection, activated lymphocytes undergo differentiation into effector cells that combat the current infection and memory cells that provide long-lasting immunity. Effector cells directly eliminate pathogens, while memory cells remember the specific pathogen for future encounters. Choices A, C, and D are incorrect as they do not accurately represent the main outcomes of clonal selection. T cell activation and cytokine suppression (A) are processes involved in immune response regulation rather than the direct outcomes of clonal selection. Pathogen destruction and tissue repair (C) are broader immune responses that occur after clonal selection. Phagocytosis and complement activation (D) involve innate immune responses, not the adaptive immune responses seen in clonal selection.

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.