What is the primary trigger for NK cell activation?

Correct Answer: B

Rationale: The primary trigger for NK cell activation is the lack of normal self-surface structures. NK cells are part of the innate immune system and recognize cells with altered or missing self-antigens. This triggers their activation to eliminate these abnormal cells. Presence of PAMPs (A) activates other immune cells like macrophages. High levels of complement proteins (C) activate the complement cascade but do not directly trigger NK cell activation. Antigen presentation by dendritic cells (D) activates adaptive immune responses through T cells, not NK cells. Therefore, the correct answer is B as it directly relates to NK cell function.

Correct Answer: C

Rationale: The correct answer is C because innate immunity plays a crucial role in initiating and influencing adaptive immunity. Innate immunity activates and stimulates adaptive immune responses by recognizing pathogens and initiating the adaptive immune system to produce specific responses. This relationship is essential for the body's overall immune defense. Choice A is incorrect because innate immunity does not suppress adaptive immunity; instead, they work together synergistically. Choice B is incorrect because adaptive immunity does not enhance innate immune functions, but rather complements them. Choice D is incorrect because innate and adaptive immunity are interconnected and work together to provide comprehensive immune protection.

Correct Answer: B

Rationale: The correct answer is B: Antigen-presenting cells (APCs). APCs, such as dendritic cells, macrophages, and B cells, are specialized cells that capture, process, and present antigens to T cells to initiate an immune response. They express major histocompatibility complex (MHC) molecules for antigen presentation. Plasma cells (A) produce antibodies, NK cells (C) are involved in killing infected cells, and eosinophils (D) are primarily involved in parasitic infections and allergic responses, not antigen presentation to T cells.

Correct Answer: C

Rationale: The correct answer is C: IgM. During the primary immune response, IgM is the first antibody produced by the immune system in response to an antigen. IgM is efficient in agglutination and complement fixation, aiding in the elimination of pathogens. IgG is more prominent in the secondary immune response. IgA is mainly found in mucosal areas. IgE is associated with allergic reactions and parasitic infections, not the primary immune response. So, IgM dominates the primary immune response by being the initial antibody produced to combat antigens.

Correct Answer: B

Rationale: The correct answer is B: Major Histocompatibility Complex (MHC). MHC molecules present antigens to T cells for immune response activation. They bind to antigenic peptides and present them on the cell surface for recognition by T cells. This is crucial for the immune system to distinguish between self and non-self antigens. Antibodies (A) are produced by B cells and do not directly present antigens to T cells. Complement protein C3 (C) is involved in the complement system for immune defense, not antigen presentation. Cytokines (D) are signaling molecules that regulate immune responses but do not directly present antigens to T cells.