Full activation of a naive T cell requires two signals. Signal 1 is provided by the T-cell receptor (TCR) binding to the peptide-MHC complex on an antigen-presenting cell (APC). Signal 2, known as costimulation, is provided by the interaction of costimulatory molecules. The most important costimulatory interaction is between the B7 molecule (CD80/CD86) on the APC and the CD28 receptor on the T cell. Blocking this interaction prevents full T-cell activation, leading to a state of unresponsiveness called anergy. This is a key target for immunosuppressive drugs.

(A) The drug does not affect the TCR-MHC interaction (Signal 1).

(B) Adhesion is mediated by molecules like LFA-1 and ICAM-1.

(D) Differentiation occurs after T-cell activation.

This clinical picture is classic for X-linked hyper-IgM syndrome, which is caused by a defect in the gene for CD40 ligand (CD40L or CD154). CD40L is expressed on activated helper T cells and is essential for interacting with the CD40 receptor on B cells. This CD40-CD40L interaction is required for B cells to undergo class switch recombination (isotype switching) from IgM to other isotypes (IgG, IgA, IgE) and to form germinal centers for affinity maturation. Without this signal, B cells can only produce IgM, leading to high levels of IgM and a deficiency of other antibodies, resulting in susceptibility to pyogenic bacteria and opportunistic pathogens.

(B) A defect in MHC class II would cause a lack of CD4+ T cells (bare lymphocyte syndrome type II).

(C) A B-cell receptor defect would lead to a lack of B cells.

(D) RAG deficiency causes severe combined immunodeficiency (SCID) with an absence of both B and T cells.

This presentation is characteristic of Leukocyte Adhesion Deficiency Type 1 (LAD-1), an autosomal recessive disorder caused by mutations in the gene encoding the β2-chain (CD18) of integrins. Integrins (like LFA-1 and Mac-1) are crucial for the firm adhesion of neutrophils to the vascular endothelium, a necessary step for extravasation into tissues. Without functional integrins, neutrophils cannot migrate from the bloodstream to sites of infection. This leads to recurrent infections without pus (which is composed mainly of dead neutrophils) and a high circulating neutrophil count (neutrophilia) because they are trapped in the blood.

(A) Selectin defects cause a similar but generally milder condition (LAD-2).

(B) Immunoglobulins are antibodies.

(C) Chemokine receptors are involved in directing migration, but the primary defect in LAD-1 is in firm adhesion.

The pre-B cell receptor checkpoint is a crucial step in B-cell development. Successful signaling from this receptor confirms that a functional heavy chain has been produced. This signal has several consequences, including inducing proliferation of the pre-B cell and, importantly, enforcing allelic exclusion by shutting down further V(D)J recombination at the immunoglobulin heavy-chain locus. This ensures that each B cell expresses a heavy chain from only one parental chromosome, maintaining clonal specificity. Following this, recombination of the light-chain genes is initiated.

(A) Heavy-chain recombination occurs before the formation of the pre-B cell receptor.

(C) Somatic hypermutation occurs much later, in mature B cells in germinal centers.

(D) Elimination of self-reactive B cells (negative selection) occurs after a complete IgM B-cell receptor is expressed.

This scenario describes a secondary (or anamnestic) immune response, which is characterized by being faster, stronger, and more effective than the primary response. This is due to the existence of long-lived memory cells generated during the primary immunization. Upon re-exposure to the tetanus antigen, pre-existing memory B cells are rapidly activated. They quickly proliferate and differentiate into plasma cells that secrete large amounts of high-affinity, isotype-switched antibodies (primarily IgG), providing rapid protection.

(A) Naive B cells would mediate a slower primary response, initially producing IgM.

(C) Plasmacytoid dendritic cells are key producers of type I interferons in viral infections.

(D) Regulatory T cells function to suppress immune responses.

Some antigens, particularly those with highly repetitive structures like bacterial capsular polysaccharides, are classified as T-independent (TI) antigens. These antigens can activate B cells without the help of T helper cells. They do so by extensively cross-linking the B-cell receptors (BCRs) on the B-cell surface, providing a strong enough signal to induce proliferation and differentiation into IgM-secreting plasma cells. This response is typically weaker, shorter-lived, generates no memory B cells, and does not involve isotype switching or affinity maturation compared to T-dependent responses.

(B) NK cells do not provide help for B-cell activation.

(C) Affinity maturation requires T-cell help in germinal centers.

(D) B cells bind intact antigen directly; they do not require processing and presentation by macrophages.

This clinical presentation is classic for Chronic Granulomatous Disease (CGD), an immunodeficiency resulting from a defect in NADPH oxidase. This enzyme complex is essential for the phagocyte respiratory burst, a process that generates superoxide radicals (O2-) from oxygen. Superoxide is then converted to other reactive oxygen species (ROS) that are toxic to ingested microbes. In CGD, phagocytes can ingest but cannot kill pathogens effectively, leading to granuloma formation and recurrent infections, particularly with catalase-positive organisms which can break down the small amount of H2O2 that the host cell might produce.

(A) Myeloperoxidase deficiency is less severe, as other killing mechanisms remain intact.

(B) G6PD deficiency can impair the respiratory burst by limiting NADPH supply, but it is primarily known for causing hemolytic anemia.

(D) Lysozyme is an enzyme in phagocytic granules that degrades bacterial cell walls.

The terminal complement components (C5-C9) assemble to form the membrane attack complex (MAC), which creates pores in the outer membranes of pathogens, leading to their lysis. The MAC is particularly important for defense against encapsulated bacteria, especially Neisseria species. Individuals with deficiencies in any of the terminal complement components (C5, C6, C7, C8, or C9) are highly susceptible to recurrent infections with Neisseria meningitidis and Neisseria gonorrhoeae.

(A, B, C) While the classical, lectin, and alternative pathways are all ways to activate the complement cascade and lead to the formation of opsonins (C3b) and anaphylatoxins (C3a, C5a), the specific vulnerability to Neisseria is most directly linked to the failure to form the lytic MAC.

During T-cell development in the thymus, thymocytes progress through several stages defined by the expression of CD4 and CD8 coreceptors. The stage at which a thymocyte expresses both CD4 and CD8 simultaneously is known as the double-positive stage. This is the stage where positive selection occurs in the thymic cortex. During positive selection, these double-positive cells are tested for their ability to recognize self-MHC. Cells that successfully recognize self-MHC will survive and differentiate into single-positive (either CD4+ or CD8+) T cells.

(A) Double-negative thymocytes are an earlier stage and lack both CD4 and CD8.

(C) Single-positive thymocytes are a later stage and express either CD4 or CD8, but not both.

(D) Mature naive T cells are single-positive cells that have exited the thymus and circulate in the periphery.

Toll-like receptor 4 (TLR4) is the primary pattern recognition receptor that recognizes lipopolysaccharide (LPS), a major component of the outer membrane of Gram-negative bacteria. Binding of LPS to TLR4 on macrophages and dendritic cells initiates a signaling cascade that leads to the production of pro-inflammatory cytokines like TNF-α, IL-1, and IL-6, causing the systemic inflammatory response seen in septic shock.

(A) TLR3 recognizes double-stranded RNA, which is associated with viral infections.

(C) NOD2 is an intracellular receptor that recognizes muramyl dipeptide, a component of bacterial peptidoglycan found in both Gram-positive and Gram-negative bacteria.

(D) RIG-I-like receptors are intracellular sensors that detect viral RNA in the cytoplasm.