CUET UG Biology Booster Test 3-Immunity and Vaccination Principles

The chronological journey of an inhaled microbe through our innate physical boundaries follows a logical path. First, the microbe attempts to bypass the body's primary physical barrier system from the outside environment (4). To do this via inhalation, it enters the nasal cavity and respiratory passages (2). Once inside these physiological conduits, it makes direct physical contact with the fluid mucus coating lining the respiratory epithelium (1). Finally, the sticky physical properties of this mucosal secretion entrap the pathogen (3), preventing it from penetrating deeper into the lungs. This creates the sequential order: 4 →2 →1 →3.

Statement I is entirely correct; the highly concentrated hydrochloric acid in the stomach creates a low pH environment that denatures proteins and destroys ingested pathogens. Statement II is correct because tears and saliva contain lysozyme, an antibacterial enzyme that actively prevents microbial proliferation. Statement III is false because physiological barriers belong to innate immunity, which is characteristically non-specific. They deploy broad chemical and enzymatic defenses that act the same way against all foreign microbes, regardless of the specific viral or bacterial strain. Therefore, only statements I and II are correct.

Matching each cellular defense element to its exact biological definition or location reveals that PMNL-neutrophils (1) match with Q, as PMNL directly stands for Polymorphonuclear Leukocytes found in blood. Macrophages (2) are large phagocytes that differentiate and reside primarily in tissues (P) to clear cellular debris and microbes. Monocytes (3) are agranular leukocytes that circulate in the blood (S), where they perform phagocytosis before migrating into tissues. Natural Killer cells (4) are specialized large granular lymphocytes that circulate in the blood (R) to destroy infected or abnormal host cells. This aligns with option D.

Interferons are inducible proteins, meaning healthy cells do not produce them preemptively (A). They are synthesized and secreted exclusively by cells that have already been hijacked by a virus (C). Once released, these signaling molecules travel to neighboring uninfected cells, stimulating them to produce antiviral proteins that block viral replication. This localized protective signaling mechanism is why they are classified as cytokine barriers (B) whose main role is protecting non-infected tissue (D).

The evolutionary advantage of adaptive or acquired immunity is its ability to remember specific pathogens. While the initial primary response is slow and resource-heavy, it generates long-lived memory B and T-lymphocytes. If the same pathogen attacks again, these memory cells enable the immune system to launch a rapid, highly intensified secondary (anamnestic) response (C). This quick response eliminates the threat before it can cause widespread disease, conserving biological energy and resources.

The primary and secondary immune responses are both branches of acquired immunity, meaning they are carried out by specialized B and T-lymphocytes. The primary response occurs during the first encounter and has a low intensity (A, B). The secondary response relies on memory cells from that first exposure to launch a faster, more intense reaction (D). Statement C is incorrect because secondary responses rely heavily on memory B and T-lymphocytes, not polymorphonuclear leukocytes (PMNLs), which are non-specific cells belonging to the innate immune barrier.

The provided passage states: "The B-lymphocytes produce an army of proteins in response to pathogens into our blood... These proteins are called antibodies." If a genetic mutation prevents B-lymphocytes from synthesizing proteins, they cannot manufacture these protective molecules. As a result, the body will experience a severe shortage or complete absence of circulating antibodies in the blood (B). The text also notes that T-cells do not secrete antibodies, meaning they cannot step in to replace this function (A).

The functional relationship between these two cell types is explicitly defined at the end of the provided text: "The T-cells themselves do not secrete antibodies but help B cells to produce them." This means T-lymphocytes act as essential helper cells within humoral immunity, providing the cellular coordination and signaling needed to activate B-lymphocytes so they can scale up antibody production (C).

Every polypeptide chain has chemical directionality, featuring an N-terminal (amino) end and a C-terminal (carboxyl) end. In an antibody structure, the antigen-binding sites are located at the tips of the "Y" shape. These tips contain the highly variable regions of both the heavy and light chains, which change shape to bind specific foreign antigens. These tips match the N-terminal ends of the protein chains. The C-terminal ends form the stable stem or constant region of the antibody structure.

The structural formula H₂L₂ (two heavy and two light chains) defines a standard antibody monomer. When B-lymphocytes secrete these antibody molecules directly into the bloodstream, they circulate throughout the body's fluids to find and neutralize specific targets. Because this defense system relies on protective proteins dissolved in body fluids (humors), it is called the humoral immune response (B). Cell-mediated immunity relies on direct T-cell action rather than secreted antibodies (A).

The sequence outlines the steps of a humoral immune response. The process begins when naive B-lymphocytes encounter and bind a specific pathogen via their membrane-bound immunoglobulin receptors (2). This interaction, along with costimulatory signals from helper T-lymphocytes, stimulates the B-lymphocytes to proliferate and differentiate into plasma cells that synthesize specific defensive proteins (4). These proteins, known as antibodies, are then released into the blood plasma and lymph (1). Finally, these circulating antibodies track down the systemic pathogen, binding to its antigenic determinants to neutralize it or mark it for destruction (3). Thus, the correct operational sequence is 2, 4, 1, 3.

Option B is not a valid justification because it incorrectly attributes antibody production to cell-mediated immunity. Circulating antibodies are the effectors of the humoral immune response, which is driven by B-lymphocytes. In contrast, cell-mediated immunity (CMI) is mediated by T-lymphocytes (CD8⁺ cytotoxic T cells and CD4⁺ helper T cells) without the use of secreted antibodies. During graft rejection, T-lymphocytes recognize foreign human leukocyte antigens (HLAs) on the transplanted tissue, release perforins and granzymes, and recruit macrophages to destroy the graft cells directly. Therefore, statement B contains a fundamental biological error, making it the correct choice for this "NOT" question.

Statement I is false because organs from other animals or primates (xenografts) cannot be easily accepted by the human body; they trigger hyperacute rejection due to species-specific structural differences and pre-existing natural antibodies. Statement II is correct; matching tissue (HLA antigens) and blood groups (ABO compatibility) is necessary to minimize immediate rejection by the host's immune system. Statement III is also correct; even with a close match, the host's cell-mediated immunity (CMI) continues to recognize the graft as foreign, posing a lifelong threat of chronic rejection. As a result, patients must take immunosuppressive drugs for life. Thus, statements II and III are correct.

The primary target of post-transplant immunosuppressive therapy (such as Cyclosporine A) is the T-lymphocyte population. Graft rejection is primarily a function of cell-mediated immunity (CMI). When an allograft is introduced, host helper T-cells (CD4⁺) identify foreign MHC markers on the graft and secrete Interleukin-2 (IL-2). This cytokine drives the clonal expansion of cytotoxic T-lymphocytes (CD8⁺), which directly attack and destroy the cells of the transplanted organ. Immunosuppressants act by blocking these T-cell activation and proliferation pathways, thereby protecting the graft from cell-mediated destruction.

Option C describes passive immunity rather than active immunity. Active immunity occurs when a host's own immune system encounters an antigen and synthesizes its own antibodies, a process that takes time but generates long-lived memory cells. In contrast, passive immunity involves introducing preformed antibodies or antitoxins directly into the patient's body from an external source. After a venomous snakebite, the toxin acts too quickly for the host to mount an active response. Injecting a preformed antitoxin provides immediate neutralization of the venom, but it does not stimulate the host's lymphocytes or create immunological memory, making it a classic example of passive immunization.

This question requires matching the physiological context to its biological medium: Infant's gut protection (1) is provided by colostrum, the initial yellowish milk secreted by the mother during the early days of lactation, which contains high concentrations of secretory IgA antibodies (Q). Foetal protection in utero (2) occurs when maternal IgG antibodies cross the placental barrier to provide passive protection to the developing fetus (R). Active humoral response (3) involves the host's own B-lymphocytes differentiating into plasma cells to manufacture antibodies within the blood plasma (S). Passive tetanus response (4) requires an immediate medical injection of preformed antitetanus antibodies or antitoxins to neutralize the bacterial toxin (P). This matches up as 1-Q, 2-R, 3-S, 4-P.

Inactivation (via heat, chemicals, or radiation) destroys a pathogen's ability to replicate and cause disease, but leaves its structural surface proteins intact. These structural elements serve as antigens. When injected, the immune system's antigen-presenting cells engulf these foreign proteins and display them to naive B and T lymphocytes. This safely triggers a primary immune response, leading to clonal expansion and the formation of specific memory B and T cells without causing clinical disease. If the host encounters the wild, fully virulent pathogen later, these memory cells launch a rapid, robust secondary response that clears the infection before symptoms appear.

During a primary immune response, there is a distinct lag phase of several days while the body selects, activates, and expands rare clones of specific naive lymphocytes. Memory B and T cells change this timeline upon re-exposure. Because the body now maintains an expanded population of these antigen-specific memory cells, the pathogen is recognized almost immediately. This significantly shortens the lag phase, skipping the slow activation steps and launching a rapid, highly intense secondary (anamnestic) response. Antibodies are produced much faster and at higher levels, neutralizing the pathogen before it can multiply and cause clinical symptoms.

Tetanus is caused by the anaerobic bacterium Clostridium tetani, which releases a highly potent neurotoxin called tetanospasmin. This toxin binds to neuromuscular junctions and travels to the central nervous system, causing painful muscle spasms and potentially fatal respiratory failure within days. Active vaccination requires introducing a toxoid to trigger the body's primary immune response, a process that takes weeks to generate protective antibody levels. In an acute exposure, the patient does not have weeks to wait. Active immunization is too slow to neutralize the rapidly acting toxin, making it necessary to provide immediate passive immunization using an injection of preformed antitetanus immunoglobulins (antitoxin).

Recombinant DNA technology represents an advancement in vaccine manufacturing. By isolating the gene that encodes a pathogen's primary surface antigen (such as the Hepatitis B surface antigen, or HBsAg) and inserting it into an expression vector within yeast (Saccharomyces cerevisiae) or bacteria, scientists can produce pure antigenic polypeptides through large-scale fermentation. This provides two major advantages: it ensures excellent safety because the vaccine contains only an isolated protein rather than a live pathogen that could mutate or cause infection, and it enables efficient, large-scale production. The Hepatitis B vaccine is a classic example of a recombinant subunit vaccine produced using yeast systems.