The clonal selection theory explains:

Correct Answer: C

Rationale: Rationale: 1. Clonal selection theory states that specific lymphocytes are activated by antigens. 2. Upon activation, these lymphocytes proliferate to form a clone of cells specific to the antigen. 3. This increases the number of lymphocytes specific for the antigen. 4. Choice C correctly aligns with the essence of clonal selection theory. Summary: A: Incorrect - Clonal selection theory does not involve a single lymphocyte expressing multiple antigen receptors. B: Incorrect - Antigens do not influence the generation of antigen receptors, but rather activate specific lymphocytes. D: Incorrect - Clonal selection theory focuses on lymphocyte response to antigens, not neutrophil increase during infections.

Correct Answer: C

Rationale: The correct answer is C because pathogens are microorganisms that specifically cause disease in a host with normal immune functions. Pathogens have mechanisms to evade or overcome the host's immune system, leading to the development of diseases. Choices A and B are characteristics of both pathogens and non-pathogenic organisms. Choice D is incorrect because not all pathogens are transmitted between individuals of the same species; some pathogens can be transmitted from animals to humans or through environmental sources.

Correct Answer: B

Rationale: The correct answer is B. Herd immunity refers to the protection offered to non-immunized individuals when a high percentage of the population is immunized, preventing the pathogen from spreading easily. This is because when a large portion of the population is immune to a disease, it creates a barrier that reduces the overall transmission of the pathogen, thus protecting those who are not vaccinated or cannot be vaccinated. Choice A is incorrect because herd immunity is not solely about the total number of immunized individuals, but rather about the impact on the entire population. Choice C is incorrect as it refers to the efficacy of the vaccine in generating a protective response, not the concept of herd immunity. Choice D is incorrect as it relates to social influence rather than the collective protection provided by high rates of immunization.

Correct Answer: A

Rationale: Rationale: 1. FOXP3 gene encodes a transcription factor crucial for regulatory T cells function. 2. Mutations in FOXP3 lead to dysfunctional regulatory T cells, causing autoimmune disorders. 3. Scurfy phenotype matches the symptoms of autoimmune disorders seen with FOXP3 mutations. Summary: - B (AIRE): AIRE mutations cause autoimmune polyendocrine syndrome, not scurfy phenotype. - C (NFkB): NFkB is a transcription factor involved in immune response but not linked to scurfy phenotype. - D (IRF): IRF is a regulator of interferon signaling, not directly associated with scurfy phenotype.

Correct Answer: D

Rationale: The correct answer is D because secondary lymphoid tissues are where both A and C processes occur. Naïve lymphocytes become activated in secondary lymphoid tissues through interactions with antigens, leading to their differentiation into effector cells. Somatic hypermutation also occurs in secondary lymphoid tissues, specifically in germinal centers of lymph nodes, leading to the generation of high-affinity antibodies. Option B, VDI gene rearrangement, does not occur in secondary lymphoid tissues; it is a process that happens in the primary lymphoid organs (bone marrow and thymus) during the development of lymphocytes. Thus, options A and C are correct, making the correct answer D.