What do pattern recognition receptors (PRRs) identify?

Correct Answer: B

Rationale: Pattern recognition receptors (PRRs) identify Pathogen-associated molecular patterns (PAMPs) because PAMPs are unique molecules found on pathogens that trigger an immune response. PRRs recognize these patterns to distinguish between self and non-self. This recognition is crucial for initiating an immune response against potential threats. Specific antigens (A) and specific viruses only (D) are too narrow in scope as PRRs are designed to detect a wide range of patterns. Host self-proteins (C) are recognized by other mechanisms in the immune system to prevent autoimmunity.

Correct Answer: C

Rationale: The correct answer is C, Lectin pathway. MBL recognizes pathogen-associated molecular patterns containing mannose, initiating the lectin pathway. MBL binds to mannose on microbial surfaces, activating MBL-associated serine proteases, leading to the formation of the C3 convertase. This triggers the rest of the complement cascade. A: Alternative pathway is triggered by spontaneous hydrolysis of C3, not by MBL. B: Classical pathway is activated by antigen-antibody complexes, not MBL. D: Terminal pathway (also known as the membrane attack complex) is the final step of all complement pathways, not specifically triggered by MBL.

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.