Which scientist proposed the concept of clonal selection?

Correct Answer: B

Rationale: The correct answer is B: Macfarlane Burnet. Burnet proposed the concept of clonal selection in the 1950s to explain how the immune system recognizes and responds to specific antigens. He suggested that lymphocytes with receptors specific to antigens undergo clonal expansion upon encountering the antigen. This hypothesis laid the groundwork for our understanding of adaptive immunity. A: Paul Ehrlich developed the side-chain theory of antibody formation, not clonal selection. C: Louis Pasteur is known for his work in microbiology and vaccination, not clonal selection. D: Elie Metchnikoff is credited with discovering phagocytosis, not clonal selection.

Correct Answer: B

Rationale: The correct answer is B because lymphocytes are activated in adaptive immunity by recognizing their specific antigen. This recognition is crucial for initiating an immune response tailored to combat a specific pathogen. Phagocytosis (A) is the process of engulfing and destroying pathogens, but it does not directly activate lymphocytes. Cytokine release by macrophages (C) can help in activating other immune cells, but lymphocytes specifically require antigen recognition for activation. Opsonization of pathogens (D) enhances phagocytosis but is not the direct trigger for lymphocyte activation.

Correct Answer: C

Rationale: The correct answer is C: IgG. In the secondary immune response, IgG antibodies predominate due to memory B cells rapidly producing them upon re-exposure to the same antigen. IgG provides long-lasting immunity and is more efficient in neutralizing pathogens. IgM (A) is the primary antibody in the initial immune response. IgE (B) is involved in allergic reactions. IgD (D) functions as a receptor on B cells.

Correct Answer: B

Rationale: The correct answer is B: Mast cells. Mast cells play a central role in Type I hypersensitivity reactions by releasing histamine, cytokines, and other inflammatory mediators in response to allergens. This triggers the allergic response. Neutrophils (A), Macrophages (C), and Dendritic cells (D) are not central to Type I hypersensitivity reactions. Neutrophils are involved in acute inflammation, macrophages play a role in phagocytosis and antigen presentation, and dendritic cells are important in initiating adaptive immune responses.

Correct Answer: A

Rationale: The correct answer is A: Multiple sclerosis. Type IV hypersensitivity involves T cell-mediated immune reactions, leading to tissue damage. In multiple sclerosis, myelin sheath destruction is caused by activated T cells attacking self-antigens. Incorrect choices: B: Grave's disease is an example of Type II hypersensitivity involving autoantibodies targeting specific cells. C: Anaphylaxis is a Type I hypersensitivity reaction mediated by IgE antibodies causing rapid and severe allergic reactions. D: Eczema is a chronic inflammatory skin condition involving complex immune mechanisms not specific to Type IV hypersensitivity.