CUET UG Biology Booster Test 2-Immune Disorders AIDS and Cancer

In healthy biological tissues, cell proliferation and differentiation are kept in a precise homeostatic balance through a process called contact inhibition. When normal cells divide and physically crowd together, surface-bound cell adhesion molecules trigger intracellular signaling pathways that arrest the cell cycle in the G₁ phase, preventing overcrowding. As detailed in the passage, cancer cells lose this regulatory mechanism. Without contact inhibition, the cells ignore density-dependent signals and divide continuously. They pile up into disorganized, multilayered structures rather than a controlled monolayer. This unchecked mitotic activity forms an abnormal tissue mass known as a tumor or neoplasm, directly validating Option B.

Tumors are categorized as either benign or malignant based on their structural growth patterns and ability to spread. Benign tumors stay restricted to their site of origin and do not invade nearby tissues. In contrast, malignant tumors are aggressive masses of neocellular tissue that grow rapidly and invade surrounding structures. Metastasis is a defining, advanced feature of malignant cancers. Cells continuously detach, or slough off, from the primary tumor mass and penetrate nearby blood vessels or lymphatic channels. These circulating tumor cells travel through the vascular network until they lodge in distant capillary beds, such as in the lungs, liver, or brain. They then migrate out of the vessels and begin dividing anew, forming destructive secondary tumors throughout the body. This systemic spread makes treatment difficult, which is why it is considered the most dangerous property of cancer.

The small, solid oval structures located along the lymphatic vessels are lymph nodes, which serve as secondary lymphoid organs. Interstitial fluid that bathes body tissues drains into lymphatic capillaries as lymph, often carrying foreign antigens, cell debris, or pathogens from local infections. As this fluid flows through the lymph nodes, their internal mesh-like structural tissue acts as a physical filter to trap these microorganisms. The nodes are packed with mature B-lymphocytes, T-lymphocytes, and antigen-presenting macrophages. When these immune cells encounter the trapped antigens, they undergo rapid activation and proliferation. This process mounts an adaptive immune response to neutralize the infection before the filtered lymph fluid is returned to the cardiovascular circulatory system.

While lymph nodes filter interstitial tissue fluid, the spleen serves as the primary filter for the cardiovascular circulatory system. The spleen is a large, bean-shaped secondary lymphoid organ located in the upper left quadrant of the abdominal cavity. It is divided structurally into red pulp and white pulp. The white pulp contains organized clusters of B and T lymphocytes that monitor the blood for systemic infections. The red pulp consists of vascular cords packed with macrophages and dendritic cells. As blood moves through this tissue, these phagocytic cells trap and destroy blood-borne microorganisms. Additionally, the spleen removes old, damaged red blood cells, acting as a graveyard for erythrocytes and a reservoir for platelets. [ SECONDARY LYMPHOID FILTERS ] / \ [ Lymph Nodes ] [ Spleen ] | | Filters Tissue Fluid Filters Circulating Blood

Statement D is factually incorrect because antibodies are soluble proteins, not cells, and therefore cannot perform phagocytosis. Phagocytosis is an active process in which specialized immune cells, such as macrophages and neutrophils, engulf and digest large foreign particles. During an allergic reaction, IgE antibodies function as biochemical adaptors. The constant (Fc) region of the IgE molecule binds to high-affinity FcεRI receptors on the surface of mast cells and basophils. When an individual is re-exposed to the allergen, the allergen binds to and cross-links these surface-bound IgE antibodies. This cross-linking signals the mast cell to degranulate, releasing inflammatory mediators such as histamine into the surrounding tissue. Because antibodies bind to antigens rather than engulfing them, statement D is incorrect and is therefore the correct answer for this negative question.

This question matches key molecules and drugs involved in allergic pathways to their functional descriptions: Histamine (1) is an inflammatory chemical mediator stored inside the granules of mast cells and basophils. When released, it binds to local vascular receptors, causing vasodilation, increased capillary permeability, and smooth muscle spasms that lead to allergy symptoms (ii). IgE (2) is the specific antibody class produced by differentiated B-cells during allergic sensitization. It circulates and binds to the surface receptors of mast cells, priming them for subsequent allergen exposure (iii). Anti-histamines (3) are competitive pharmacological antagonists that bind to and block H₁ histamine receptors on target tissues, thereby quickly reducing allergy symptoms such as swelling, sneezing, and itching (i). This yields the correct matching layout: 1-ii, 2-iii, 3-i.

Higher vertebrates have evolved an advanced, memory-based adaptive immune system that relies on distinguishing between "self" markers (the body's own healthy proteins and tissues) and "non-self" markers (foreign pathogens, toxins, and abnormal cells). This self-tolerance is established during lymphocyte development. Immature T and B cells that react too strongly to self-antigens are eliminated or inactivated through clonal deletion and anergy. An autoimmune disease occurs when this self-tolerance mechanism breaks down due to genetic mutations, environmental triggers, or structural similarities between foreign antigens and self-proteins (molecular mimicry). The immune system misidentifies host tissues as foreign threats and mounts an attack against them using autoreactive T-cells and autoantibodies. This represents a failure to differentiate foreign organisms from self-cells, validating Option B.

Statement I is correct; rheumatoid arthritis (RA) is a chronic inflammatory disorder in which the immune system mistakenly attacks the body's own tissues. The body produces autoantibodies, such as Rheumatoid Factor (RF), that target its own immunoglobulins, leading to the accumulation of immune complexes in the joints. Statement II is also correct; RA is a classic and common example of an autoimmune disease in humans, causing inflammation, thickening of the synovial membrane, and progressive cartilage damage. Statement III is incorrect; RA is not caused by an active retroviral infection of the joints. Although environmental factors such as smoking or previous viral exposures may contribute to the development of the disease in genetically predisposed individuals, the joint damage itself results from a persistent autoimmune response rather than an active viral infection. Therefore, only Statements I and II are correct.

Lymphoid organs are divided into primary and secondary structures based on their role in lymphocyte development. Bone marrow and the thymus are classified as primary lymphoid organs. These tissues provide specialized microenvironments needed for the early development of immune cells. Inside the bone marrow, hematopoietic stem cells divide and give rise to immature lymphoid progenitors. For B-lymphocytes, the entire process of development, structural gene rearrangement, and maturation occurs within the bone marrow tissue. For T-lymphocytes, immature progenitors travel from the bone marrow to the thymus to complete maturation. In both organs, immature cells mature into functional, antigen-sensitive lymphocytes that express specific surface receptors before entering circulation. This makes Option B the correct explanation. [ PRIMARY LYMPHOID ORGANS ] -----> Maturation & Differentiation (Bone Marrow / Thymus) | v [ SECONDARY LYMPHOID ORGANS ] ----> Antigen Interaction & Proliferation (Spleen / Lymph Nodes)

Statement C is factually incorrect because the thymus does not keep growing past puberty; instead, it undergoes a process called age-related involution. The thymus is a bilobed primary lymphoid organ located in the chest, sitting just behind the sternum (breastbone) and in front of the heart. It provides the specialized microenvironment of epithelial cells and cytokines needed for immature T-lymphocyte progenitors to develop into mature, functional T-cells. The thymus is largest relative to body weight at birth and reaches its absolute maximum physical weight during puberty. After puberty, sex hormone production triggers age-related involution, causing the functional cortical and medullary tissues to gradually shrink and be replaced by fatty adipose tissue. This leads to a decline in the output of new naive T-lymphocytes in older adults. Because statement C describes a pattern of continuous growth that does not occur, it is the correct choice for this "NOT" question.

• Structurally, a retrovirus particle like HIV is built in concentric layers. Looking from the inside out: Innermost Core (2): Consists of the viral RNA genome along with essential enzymes like reverse transcriptase. Middle Layer (3): The genetic material is housed directly within an icosahedral protein coat (capsid). Outermost Layer (1): Enclosing the protein coat is the viral envelope, a lipid bilayer derived from the host cell's plasma membrane embedded with viral glycoproteins. Arranging these architectural components from innermost to outermost gives the sequence 2 →3 →1, matching option A.

• HIV is not a highly contagious airborne or casual contact virus. It spreads through specific fluids like blood, semen, and vaginal secretions. Statements I, II, and III represent well-established transmission routes: multiple sexual partners increase exposure risk; shared needles expose drug users to infected blood; and frequent transfusions increase the probability of encountering contaminated blood units. Statement IV is false: HIV cannot survive outside the body on dry utensils, nor is it transmitted through saliva via sharing plates or cups. Casual, everyday social interactions pose zero risk. Therefore, only statements I, II, and III are valid high-risk factors.

• The defining feature of a retrovirus is its ability to reverse the standard flow of genetic information (which normally moves from DNA to RNA). Once the HIV core enters a host cell, it uses its viral enzyme, reverse transcriptase, to rewrite its single-stranded viral RNA into double-stranded viral DNA. If a pharmacological agent or mutation completely blocks this enzyme, this conversion fails entirely. The virus cannot generate viral DNA, preventing it from integrating into the host's genome or producing new viral copies.

• When HIV enters the human body, it targets cells expressing the CD4 receptor, primarily macrophages and helper T-lymphocytes (TH cells). Inside macrophages, the virus converts its RNA into DNA, integrates into the host genome, and directs the cell to assemble new virions. Crucially, macrophages can tolerate viral replication without dying immediately. They survive for extended periods, functioning as persistent reservoirs that continuously release new viruses, earning them the nickname "HIV factories." Statement C claims that macrophages are destroyed immediately, which is factually incorrect and therefore the correct choice for this "NOT true" question.

Helper T-lymphocytes (CD4⁺ T-cells) act as the central coordinators of adaptive immunity, releasing cytokines that activate both B-cells (for antibody production) and cytotoxic T-cells (for cell-mediated defense). As HIV infection progresses, the virus systematically infects and destroys these helper T-cells. Over time, their numbers decline drastically. This loss leaves the immune system severely compromised, resulting in a profound state of immunodeficiency. Common environmental pathogens—such as Mycobacterium, Toxoplasma, fungi, and viruses—that are normally controlled by a healthy immune system can then cause serious and potentially life-threatening opportunistic infections.

• The natural history and clinical presentation of HIV follows a predictable timeline: Step 1: Following exposure, there is a distinct time-lag or incubation period before clear signs appear (ranging from a few months up to 5–10 years). Step 2: During this asymptomatic window, the virus actively replicates inside the lymphoid organs, causing a progressive decline in the host's helper T-lymphocyte count. Step 3: Once the T-cell count falls low enough, the patient develops symptomatic disease, experiencing recurrent fever, chronic diarrhea, and unexplained weight loss. Step 4: The patient seeks medical care, and the disease is formally identified using the standard ELISA screening assay. This logical clinical progression corresponds perfectly to the sequence 1, 2, 3, 4.

• Carcinogens are agents that transform normal cells into cancerous ones. They are categorized by their nature: 1. Physical ionizing radiation matches iii (X-rays): High-energy waves that penetrate deeply and break DNA strands. 2. Physical non-ionizing radiation matches ii (UV rays): Lower-energy waves that damage DNA by causing pyrimidine dimers, often leading to skin cancers. 3. Chemical carcinogen matches iv (Tobacco smoke): Contains harmful chemical compounds that are a leading cause of lung cancer. 4. Biological agent matches i (Oncogenic viruses): Viruses that carry cancer-causing genes (viral oncogenes) into host cells. Matching these up gives 1-iii, 2-ii, 3-iv, 4-i, which corresponds to option C.

• The human genome contains normal genes called proto-oncogenes or cellular oncogenes (c-onc). These genes normally regulate regular cell growth and division. However, if they are mutated or activated abnormally by environmental carcinogens, they can drive the cell toward uncontrolled, cancerous growth. In contrast, viral oncogenes are foreign, cancer-inducing genes carried within the genetic payload of oncogenic viruses. When these viruses infect a host cell, they insert these viral oncogenes to directly force neoplastic transformation. Option B outlines this genetic distinction.

• Magnetic Resonance Imaging (MRI) is a diagnostic technique used to safely image internal tissues. It relies on a strong magnetic field and non-ionizing radiofrequency pulses to change the alignment of hydrogen nuclei in the body. It does not use ionizing radiation (like X-rays or gamma rays), which makes it a safer choice for repeated scans. Because Option C states that MRI uses ionizing radiation, it is factually incorrect and the correct answer for this "NOT correctly matched" question.

• Cancer cells are mutated versions of the body's own cells, allowing them to dodge detection by the immune system. To counter this camouflage, cancer therapies often include immunotherapy alongside surgery or chemotherapy. α-interferon is a natural biological response modifier. When given to a patient, it stimulates natural killer (NK) cells, macrophages, and T-lymphocytes. This activation helps the immune system spot the hidden cancer cells and mount an effective defense to destroy the tumor.