What type of cells do natural killer (NK) cells target?

Correct Answer: B

Rationale: Natural killer (NK) cells target virus-infected and cancerous cells because they have the ability to recognize abnormal cells through activating receptors. These cells can identify changes in surface markers associated with viral infection or cancer, leading to their destruction. NK cells do not target bacterial cells (choice A), plasma cells (choice C), or self-antigens (choice D) as these are not typically recognized as threats by the immune system. NK cells play a crucial role in immune surveillance against abnormal cells, making choice B the correct answer.

Correct Answer: B

Rationale: The primary role of phagocytosis in innate immunity is to engulf and digest pathogens. Phagocytes such as macrophages and neutrophils recognize and engulf bacteria, viruses, and other harmful substances to prevent infections. They digest these pathogens using enzymes and destroy them. This process is crucial in the initial defense against infections. Option A is incorrect because phagocytosis primarily targets pathogens, not cancer cells. Option C is incorrect because producing antibodies is a function of adaptive immunity, not innate immunity. Option D is incorrect because activating complement proteins is a separate mechanism in the immune response.

Correct Answer: B

Rationale: The correct answer is B: TLR4. TLR4 is primarily responsible for recognizing lipopolysaccharide (LPS) found in the outer membrane of Gram-negative bacteria. It forms a complex with MD-2 and CD14 to trigger downstream signaling pathways. TLR2 recognizes other microbial components such as lipoproteins. TLR5 recognizes flagellin, a component of bacterial flagella. TLR9 recognizes unmethylated CpG DNA motifs. Therefore, TLR4 is specifically involved in detecting LPS, making it the correct answer in this scenario.

Correct Answer: B

Rationale: The correct answer is B: Clonal selection. During clonal selection, a specific lymphocyte recognizes its specific antigen, binds to it, and undergoes proliferation to generate a large population of identical cells to combat the antigen efficiently. This process ensures an effective immune response. Incorrect choices: A: Clonal deletion - This process involves the elimination of self-reactive lymphocytes to prevent autoimmunity, not the proliferation of antigen-specific lymphocytes. C: Antigen presentation - This is the process by which antigens are displayed to immune cells for recognition, not the proliferation of lymphocytes. D: Cytokine signaling - While cytokines play a role in immune responses, they are not directly involved in the proliferation of lymphocytes in response to specific antigens.

Correct Answer: B

Rationale: The correct answer is B: IgG. IgG is the most abundant antibody isotype in circulation, constituting about 75% of total antibodies. This is because IgG is produced during secondary immune responses and has a longer half-life compared to other isotypes, allowing it to accumulate in circulation. IgA is mainly found in mucosal surfaces, IgE is associated with allergic reactions, and IgM is the first antibody produced in a primary immune response. Therefore, IgG is the most abundant in circulation due to its longevity and role in secondary immune responses.