Complement fixation refers to

Correct Answer: C

Rationale: Complement fixation refers to the binding of complement components by antigen-antibody complexes. This process involves the activation of the complement pathway, leading to the formation of the membrane attack complex and subsequent lysis of target cells. Choice A is incorrect as it describes opsonization, not complement fixation. Choice B is incorrect as it refers to heat inactivation of complement. Choice D is incorrect as it does not involve complement fixation but rather describes the interaction of C3a with mast cells.

Correct Answer: C

Rationale: Rationale for Correct Answer (C): When spleen cells from a donor mouse are injected into a genetically dissimilar recipient mouse, it can lead to a graft-versus-host reaction. This is because the recipient's immune system recognizes the donor cells as foreign and mounts an immune response against them. In this case, since the donor mouse is of strain A and the recipient is an F1 hybrid of strains A and B, the genetic disparity is significant, leading to a higher likelihood of a strong immune response from the recipient against the donor spleen cells. Summary of Incorrect Choices: A: The spleen cells being destroyed is less likely as the recipient's immune system would typically recognize and respond to foreign cells rather than simply destroy them. B: Spleen cells surviving without any effect is unlikely because of the genetic disparity between the donor and recipient mice. D: Inducing tolerance of strain A grafts in the recipient is less likely as a strong immune response is expected due to the genetic differences between

Correct Answer: B

Rationale: The correct answer is B: Individuals of blood group type AB are 'universal recipients' of transfusions. This is because individuals with blood type AB have both A and B antigens on their red blood cells, making them compatible with all blood types during transfusions. Choice A is incorrect because Rh factor is separate from ABO blood group system. Choice C is incorrect because individuals with blood type AB do not have circulating anti-A or anti-B antibodies. Choice D is incorrect because haplotype refers to a set of genes on one chromosome, which is not directly related to blood group type.

Correct Answer: B

Rationale: The correct answer is B because if DNA synthesis is not stimulated in the mixed-lymphocyte culture, it indicates that person Z has at least one allele that is different from person X's HLA-Dw7 allele. This rules out person Z being homozygous for HLA-Dw7 (choice A) as there would be no difference in HLA alleles between the two individuals. Similarly, person Z being heterozygous for HLA-Dw7 (choice C) is too specific as it assumes only one specific allele is different. Choice D is incorrect as it states that person Z does not carry the HLA-Dw7 allele, which cannot be concluded definitively from the information provided. Therefore, the most reasonable conclusion is that person Z is either homozygous or heterozygous for HLA-Dw7 (choice B) since there is a lack of DNA synthesis stimulation, indicating a difference in HLA alleles between the two individuals.

Correct Answer: B

Rationale: The correct answer is B: C3b. C3b is a key opsonin that binds to pathogens and enhances their recognition and ingestion by phagocytes. Macrophages and neutrophils have receptors for C3b, allowing them to efficiently phagocytose pathogens. Factor D, C6, and properdin are not directly involved in enhancing phagocytosis through opsonization. Factor D is part of the alternative pathway of complement activation, C6 is involved in forming the membrane attack complex, and properdin stabilizes the alternative pathway C3 convertase. Therefore, the presence of receptors for C3b on phagocytes is crucial for their ability to recognize and engulf pathogens effectively.