Which complement protein is the main component of the Membrane Attack Complex (MAC)?

Correct Answer: C

Rationale: The correct answer is C: C5b. The Membrane Attack Complex (MAC) is formed by a sequence of complement proteins, with C5b playing a crucial role as the initiator. After C5b binds to the target cell membrane, it triggers the assembly of C6, C7, C8, and multiple C9 molecules to form the MAC pore. C1 (A) is the initiator of the classical pathway, not a component of the MAC. C3 (B) is involved in the formation of the C3 convertase and opsonization but is not the main component of the MAC. C4b (D) is part of the classical pathway and is involved in the formation of the C3 convertase, not the MAC.

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.

Correct Answer: C

Rationale: The correct answer is C: Neutralization, opsonization, and complement activation. Antibodies play a crucial role in adaptive immunity by neutralizing pathogens, facilitating their destruction through opsonization, and activating the complement system for pathogen elimination. This is achieved through binding to antigens on pathogens, marking them for destruction by phagocytes, and triggering the complement cascade. Options A, B, and D are incorrect because antibodies do not directly present antigens, perform phagocytosis, or kill infected cells.