Q.What are the non-specific (innate) lines of defence of the body?
Concept understanding — Non-Specific (Innate) Immunity — Barriers and Lines of Defence
Non-specific, or innate, immunity is the immune system's general-purpose, always-ready defence. It does not need to recognise a specific pathogen in advance, and it is traditionally described as acting in two lines, built on a broader set of four kinds of barrier.
The first line of defence consists of external barriers that try to stop a pathogen from ever entering the body at all — the skin, mucus lining the respiratory and digestive tracts (which traps particles before they can penetrate deeper), and secretions such as saliva and tears, which contain the enzyme lysozyme that breaks down bacterial cell walls.
The second line of defence comes into play once a pathogen has actually breached the first line: phagocytic cells that engulf the invader, natural killer (NK) cells that destroy infected or abnormal cells, the inflammatory response (localised redness, heat, swelling and pain as blood vessels dilate and immune cells are recruited to the site), and fever (a body-wide rise in temperature that makes conditions less favourable for many pathogens and speeds up immune cell activity).
Looked at more broadly, these defences fall into four types of barrier: physical barriers (the skin and the mucous membranes lining internal tracts); physiological barriers (body secretions such as the acidic pH of the stomach, and antimicrobial substances in saliva and tears such as lysozyme); cellular barriers (phagocytic white blood cells such as neutrophils and macrophages, and NK cells, which engulf or destroy pathogens directly); and cytokine barriers (virus-infected cells secrete signalling proteins called interferons, which warn and protect neighbouring, still-uninfected cells from the same virus).
A newborn baby's own immune system is still immature at birth, so it depends for a while on ready-made protection passed on by the mother. Colostrum, the thick, antibody-rich first milk produced in the days just after birth, is especially important because it is loaded with antibodies (mainly IgA), which coat the lining of the infant's gut and give it immediate, passive protection against infections until its own immune system is mature enough to take over.
The two non-specific lines of defence are external barriers (first line) and internal cellular/inflammatory responses (second line).
First line — external barriers such as skin, mucus and lysozyme-containing secretions. Second line — phagocytes, NK cells, inflammation and fever, once a pathogen has breached the first line.
Non-specific (innate) immunity is organised into two lines. The first line of defence is made of external barriers that try to stop a pathogen from entering the body at all — the skin, mucus lining the respiratory and digestive tracts, and secretions such as saliva and tears containing the antibacterial enzyme lysozyme. The second line of defence comes into action once a pathogen has actually breached the first line, and includes phagocytic cells that engulf the invader, natural killer (NK) cells that destroy infected cells, the inflammatory response (redness, heat, swelling and pain at the site) and fever (a body-wide rise in temperature).
First line = external barriers (skin, mucus, lysozyme-containing secretions); second line = phagocytes, NK cells, inflammation and fever.
- Confusing the innate "lines of defence" with the four barrier types (physical/physiological/cellular/cytokine) — they are two different, overlapping ways of classifying the same innate defences.
Showing the 12 most recent of 20 on this concept.
- CBSE 2026Set A1 markMCQQ.What kind of barrier is the interferon secreted by virus infected cells?(a) Cytokine barrier(b) Physical barrier(c) Cellular barrier(d) Physiological barrier
›Reveal solutionSolution
Interferons are proteins (cytokines) released by virus-infected cells, forming the cytokine barrier of innate immunity.
Innate immunity has four kinds of barriers: physical (skin, mucus), physiological (acid in stomach, saliva, tears), cellular (phagocytes like macrophages, neutrophils) and cytokine barriers. Virus-infected cells secrete proteins called interferons, which protect neighbouring non-infected cells from further viral infection. Because interferons are cytokines, they constitute the cytokine barrier.
✓Final answer(A) Cytokine barrier.
- CBSE 2025Set ANNUAL1 markMCQQ.Virus-infected cells secrete proteins called interferons which protect non-infected cells from further viral infection is called(a) cellular barrier(b) physiological barrier(c) physical barrier(d) cytokine barrier
›Reveal solutionSolution
Interferons are the classic example of the cytokine barrier component of innate immunity.
Innate immunity provides four types of barriers: physical (skin, mucus), physiological (stomach acid, saliva, tears), cellular (phagocytic cells, NK cells), and cytokine barriers. When a cell is infected by a virus, it secretes small signalling proteins called interferons. These interferons diffuse to and act on neighbouring, still-uninfected cells, inducing in them an "antiviral state" that protects them from being infected by the same virus. This mechanism — protection of surrounding cells via a secreted signalling protein — is specifically classified as the cytokine barrier of innate immunity, distinct from the purely physical, chemical (physiological), or cell-mediated (cellular) barriers.
✓Final answer(d) cytokine barrier
- CBSE 2025Set ANNUAL1 markMCQQ.A cytokine barrier, which is an innate immunity, depends on the production of certain substance by an infected cells that protects other cells from further infection. Identify this substance -(i) Histamine(ii) Serotonin(iii) Interferon(iv) Phagocyte
›Reveal solutionSolution
Virus-infected cells secrete interferons, which protect neighbouring, still-uninfected cells from being infected by the same virus — this is the cytokine barrier of innate immunity.
Innate immunity includes several types of physiological/cellular barriers, one of which is the cytokine barrier. When a cell is infected by a virus, it secretes proteins called interferons. These interferons diffuse to and act on nearby uninfected cells, inducing an antiviral state in them (by interfering with viral replication machinery), so that even if these neighbouring cells subsequently get exposed to the virus, they are protected from further infection.
Histamine and serotonin are inflammatory mediators (released mainly by mast cells/basophils, causing vasodilation and other inflammation symptoms), and phagocytes are cells (not a secreted substance) that engulf pathogens — neither fits the description of a secreted antiviral substance forming a cytokine barrier.
✓Final answerThe substance is Interferon (Option iii).
- CBSE 2024Set 57/2/11 markMCQQ.Assertion (A) : Virus-infected cells produce interferons. Reason (R) : Interferons can cause inflammation of virus-infected cells. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Assertion (A) is false, but Reason (R) is true.
›Reveal solutionSolution
Assertion (A) is true because virus-infected cells produce interferons as an antiviral defense. Reason (R) is false because interferons primarily induce an antiviral state in neighboring cells and activate immune cells, rather than directly causing inflammation of the infected cells themselves. The correct option is (C).
Concept and Intuition
Our bodies have a sophisticated defense system against pathogens like viruses. When a cell gets infected by a virus, it's not a passive victim; it actively tries to alert its neighbors and the broader immune system. One crucial way it does this is by producing signaling molecules called interferons.
Interferons are a type of cytokine (cell-signaling protein) that play a vital role in the innate immune response, particularly against viral infections. Their primary function is to "interfere" with viral replication. They achieve this mainly by:
- Signaling to uninfected cells: Interferons bind to receptors on nearby healthy cells, prompting them to produce antiviral proteins. These proteins can then block viral replication if those cells become infected. This is like putting up a protective shield around healthy cells.
- Activating immune cells: Interferons also activate various immune cells, such as Natural Killer (NK) cells and macrophages, which are crucial for destroying virus-infected cells and clearing the infection.
- Modulating immune responses: They can also influence the adaptive immune system.
Inflammation, on the other hand, is a complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. It's characterized by redness, swelling, heat, pain, and loss of function. While interferons are part of the broader immune response that can contribute to inflammation by activating immune cells, their direct action is not to cause inflammation of the virus-infected cells themselves. Instead, they focus on protecting uninfected cells and orchestrating the elimination of infected ones.
Step-by-Step Evaluation
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Evaluate Assertion (A): "Virus-infected cells produce interferons."
- When a virus infects a cell, the cell recognizes viral components (like viral RNA or DNA). This recognition triggers a signaling pathway within the infected cell, leading to the production and secretion of interferons (specifically Type I interferons, such as IFN-alpha and IFN-beta). These interferons are then released into the extracellular space.
- This statement accurately describes a fundamental aspect of the innate antiviral immune response.
- Therefore, Assertion (A) is true.
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Evaluate Reason (R): "Interferons can cause inflammation of virus-infected cells."
- As discussed, the primary role of interferons is to induce an antiviral state in neighboring uninfected cells and to activate other immune cells to clear the infection. They do not directly cause inflammation of the virus-infected cells.
- While interferons can contribute to the overall immune response that includes inflammation (e.g., by activating macrophages that release pro-inflammatory cytokines), their direct mechanism of action is not to inflame the infected cells themselves. The infected cells are often targeted for destruction by other immune cells (like NK cells or cytotoxic T lymphocytes) or undergo apoptosis (programmed cell death), rather than being directly inflamed by interferons.
- Therefore, Reason (R) is false.
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Determine the relationship between (A) and (R).
- Since Assertion (A) is true and Reason (R) is false, the question of whether R is a correct explanation for A becomes irrelevant.
Watch outIt's a common misconception to conflate all immune responses with direct inflammation. While interferons are crucial immune mediators, their specific role is antiviral signaling and immune cell activation, not directly causing the cardinal signs of inflammation in the infected cells themselves.
Interferons (IFNs) are cytokines that primarily mediate antiviral and immunomodulatory functions.
ImportantVirus-infected cells produce interferons to protect neighboring cells and activate the broader immune system.
✓Final answerAssertion (A) is true, but Reason (R) is false, making the correct option (C).
- CBSE 2024Set E1 markMCQQ.Which of the following blood cells is known as PMNL?(a) Lymphocyte(b) Monocyte(c) Neutrophil(d) Platelets
›Reveal solutionSolution
Neutrophils are the polymorphonuclear leukocytes (PMNL), named for their many-lobed nucleus.
White blood cells are grouped by nuclear shape:
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Neutrophils have a nucleus with several (poly-) lobes, so they are called POLYMORPHONUCLEAR leukocytes (PMNL). They are phagocytic cells of the innate immune system and are the most abundant WBC.
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Lymphocytes and monocytes have a single, rounded/indented nucleus (mononuclear), so they are not PMNL. Platelets are cell fragments, not leukocytes.
✓Final answer(c) Neutrophil.
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- CBSE 2024Set D1 markQ.Write True or False: Each interferon has its own speciality.
›Reveal solutionSolution
The statement is True — interferons are not a single uniform substance; each type has its own distinctive characteristics.
Interferons are proteins secreted by virus-infected cells as part of the innate/non-specific immune response. They diffuse to and bind to neighbouring, uninfected cells, inducing those cells to produce substances (antiviral proteins) that inhibit viral replication, thereby protecting them from further infection.
Interferons are broadly classified into types (such as alpha, beta, and gamma interferons in humans), and each type/kind of interferon has its own particular speciality — differing in the cells that produce them, the receptors they act through, and the precise antiviral/immune-modulating effects they exert.
This property has also been exploited biotechnologically — recombinant interferons (e.g., produced by genetically engineered microbes) are used as antiviral and anti-cancer therapeutic agents.
✓Final answerTrue.
- CBSE 2024Set D1 markQ.Answer in one sentence: How do cytokine barriers provide innate immunity in humans?
›Reveal solutionSolution
Cytokine barriers involve virus-infected cells secreting interferons that protect nearby, uninfected cells from being infected by the same virus.
Innate immunity is the non-specific type of defence present in the body from birth, and it operates through four types of barriers: physical barriers (e.g., skin, mucus lining), physiological barriers (e.g., acid in the stomach, saliva, tears), cellular barriers (e.g., phagocytes, natural killer cells), and cytokine barriers.
The cytokine barrier specifically refers to the action of interferons — small proteins secreted by cells that have been infected by a virus. Once released, interferons diffuse out and bind to receptors on nearby, still-healthy (uninfected) cells.
This binding triggers those neighbouring cells to produce antiviral proteins, priming them to resist infection by the same virus, thereby limiting the spread of the infection through the tissue — even before the specific (adaptive) immune response has had time to act.
✓Final answerVirus-infected cells release interferons, which protect neighbouring uninfected cells from further viral infection.
- CBSE 2024Set ANNUAL1 markMCQQ.Virus-infected cells secrete proteins called ................ which protect non-infected cells from further viral infection.(a) exon(b) intron(c) interferon(d) neutrophils
›Reveal solutionSolution
When a cell is infected by a virus, it secretes interferons, small proteins that alert and protect nearby healthy cells from getting infected.
Interferons are part of the body's innate, non-specific antiviral defence. A virus-infected cell releases interferons which diffuse to neighbouring cells and induce them to produce antiviral proteins that interfere with viral replication - so even if the virus reaches those cells, its multiplication is blocked. Exons and introns are parts of gene/RNA structure (coding and non-coding sequences), not secreted proteins, and neutrophils are phagocytic white blood cells, not an interferon-type signalling protein.
✓Final answer(c) Interferon.
- CBSE 2024Set ANNUAL1 markMCQQ.Assertion (A) : Interferons are a type of antibodies produced by bacteria infected cells of the body. Reason (R) : Interferons stimulate inflammation at the site of injury.(a) Assertion (A) and Reason (R) both are true and Reason (R) is the correct explanation of Assertion (A).(b) Assertion (A) and Reason (R) both are true, but Reason (R) is not the correct explanation of Assertion (A).(c) Assertion (A) is true, but Reason (R) is false.(d) Assertion (A) and Reason (R) both are false.
›Reveal solutionSolution
Interferons are not antibodies, and their chief role is antiviral protection of neighbouring cells, not stimulating inflammation.
Interferons are a special class of proteins secreted by virus-infected cells (not merely bacteria-infected cells) as part of the innate/cell-mediated defence response — they are NOT antibodies, which are a distinct class of proteins (immunoglobulins) produced by plasma cells (B-lymphocytes) as part of humoral/specific immunity. So the Assertion is false. Interferons' principal function is to protect uninfected neighbouring cells from further viral infection by inducing an antiviral state, rather than primarily stimulating inflammation at an injury site — so the Reason as stated is also false.
✓Final answer(d) Assertion (A) and Reason (R) both are false.
- CBSE 2024Set ANNUAL1 markMCQQ.The protein which is secreted by virus-infected cells to protect non-infected cells is called(a) antigen(b) antibody(c) antibiotic(d) interferon
›Reveal solutionSolution
Interferons are antiviral proteins secreted by virus-infected cells that help neighbouring healthy cells resist infection.
When a cell is infected by a virus, it can secrete small signalling proteins called interferons, which diffuse to neighbouring, still-uninfected cells and induce in them an antiviral state (by activating enzymes that interfere with viral replication), protecting them against infection by the same or related viruses. Antigens (a) are the foreign molecules that trigger an immune response, antibodies (b) are proteins produced by plasma cells to bind and neutralise specific antigens, and antibiotics (c) are drugs that act against bacteria, not viruses.
✓Final answer(d) interferon
- CBSE 2024Set ANNUAL1 markQ.Virus infected cells provide innate immunity to other uninfected cells. Analyse.
›Reveal solutionSolution
Interferons secreted by virus-infected cells alert and protect surrounding healthy cells, providing a rapid, non-specific innate immune defence against the spread of viral infection.
When a cell is infected by a virus, it produces and secretes a class of proteins called interferons. These are released into the surrounding tissue and bind to receptors on neighbouring, uninfected cells, inducing an antiviral state in them — such cells produce proteins/enzymes that interfere with viral replication (e.g., by degrading viral RNA or inhibiting protein synthesis), so that even if they are subsequently exposed to the virus, infection and replication are blocked or greatly reduced. This response is rapid, occurs without recognising a specific antigen, and needs no prior exposure, which is why it is classified as part of innate (non-specific), rather than antigen-specific acquired, immunity.
✓Final answerThrough secretion of interferons by infected cells, which protect neighbouring uninfected cells from the virus — a non-specific, innate immune mechanism.
- CBSE 2024Set ANNUAL1 markQ.State the examples of physical barriers in innate immunity
›Reveal solutionSolution
Skin and mucus-secreting epithelial linings act as the body's first-line physical barriers against pathogen entry.
Innate immunity is a non-specific type of defence present from birth, made up of four types of barriers — physical, physiological, cellular and cytokine barriers. The physical barriers prevent entry of micro-organisms into the body and include:
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The skin, which forms the outermost, mechanically tough covering of the body, preventing entry of most pathogens.
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The mucus coating/secretions of the epithelium lining internal tracts that are in contact with the external environment — the respiratory tract, the gastro-intestinal tract and the urogenital tract — which trap invading microbes and prevent their entry into deeper tissues.
✓Final answerThe skin and the mucus lining of the respiratory, gastrointestinal and urogenital tracts are examples of physical barriers in innate immunity.
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