Q.Mention the chemical nature of an antibody and name the type of cells they are produced by. Write the difference between active and passive immune responses on the basis of antibodies.
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🔒 Start your 14-day free trial to unlock the full solution →Part (a)Concept understanding — Structure of an Antibody (Immunoglobulin)
Structure of an Antibody (Immunoglobulin)
An antibody (immunoglobulin) is a protein produced by plasma cells in response to an antigen. Each antibody molecule has a characteristic Y-shaped structure built from four polypeptide chains held together by inter-chain disulfide (–S–S–) bridges.
The four chains are of two kinds:
- Two long (heavy) chains, usually written H, which run from the tips of the arms down through the central stem of the Y.
- Two short (light) chains, usually written L, which lie along the outer edge of each arm.
Because of this composition, an antibody is represented as H2L2. The two arms end in identical antigen-binding sites (the clefts at the tips), which is why each antibody can bind two antigen molecules; the amino (N) terminals lie at the arm tips and the carboxyl (C) terminals lie at the base of the stem. …
Part (b)Concept understanding — HIV Mechanism Immune Deficiency
Let’s start with an everyday picture. Think of your body as a country, and your immune system as its army and police force. This army is always on patrol, looking for invaders — viruses, bacteria, fungi — and destroying them before they can cause trouble. Now imagine a spy so clever that it doesn’t just attack the army; it actually enters the army’s command centre, takes over the generals, and slowly disables the entire defence system. That is exactly what the HIV virus does.
HIV stands for Human Immunodeficiency Virus. The key word is immunodeficiency — a deficiency, or shortage, of immunity. The virus does not kill you directly. Instead, it systematically destroys the very cells that coordinate your body’s defence, leaving you defenceless against even mild infections.
The precise mechanism
The immune system has many types of soldiers. The most important commander cell is the helper T-cell (also called CD4+ T-cell). This cell does not kill germs itself; it sends signals that activate other cells — the killer T-cells and antibody-producing B-cells — to attack.
HIV has a special protein on its surface that fits perfectly into a receptor on the helper T-cell, like a key into a lock. Once inside, the virus hijacks the cell’s machinery. It forces the cell to produce thousands of copies of the virus. Eventually, the cell bursts open and dies, releasing new viruses that go on to infect more helper T-cells.
Over months or years, the number of helper T-cells in the blood drops steadily. When the count falls below a critical level (around 200 cells per microlitre of blood), the immune system can no longer mount a proper defence. At this stage, the person is said to have AIDS — Acquired Immunodeficiency Syndrome.
HIV causes immune deficiency by destroying helper T-cells, the master switches of the immune system. The body does not die from HIV itself, but from opportunistic infections — diseases that a healthy immune system would easily stop, such as tuberculosis, pneumonia, or certain cancers.
Why it matters
A person with HIV can look and feel healthy for many years. During this asymptomatic phase, the virus is still active, but the body is producing new T-cells to replace the ones being destroyed. This is a losing battle — eventually, production cannot keep up.
The virus is transmitted only through specific routes:
- Unprotected sexual contact
- Sharing of infected needles (common among drug users)
- From an infected mother to her child during pregnancy, childbirth, or breastfeeding
- Transfusion of infected blood (rare now due to screening)
It is not transmitted by casual contact — shaking hands, sharing food, mosquito bites, or living in the same room. …
Part (a)
- Chemical nature of an antibody: an antibody is a protein (glycoprotein) called an immunoglobulin, made of four polypeptide chains (two light + two heavy) in a “Y” shape, written H₂L₂.
- Produced by: plasma cells (activated/differentiated B-lymphocytes).
- Active vs passive on the basis of antibodies: in active immunity the person's own body produces the antibodies (after infection or vaccination), giving slow but long-lasting protection with memory; in passive immunity ready-made antibodies are supplied from outside (e.g. colostrum, anti-venom), giving immediate but short-lived protection with no memory. …
Part (a): Antibodies are Y-shaped immunoglobulin proteins (two heavy + two light chains, H₂L₂) secreted by plasma cells (B-lymphocytes); active immunity is self-produced antibody (lasting, with memory) while passive immunity is externally supplied antibody (immediate, temporary). Part (b): HIV uses helper-T (CD4⁺) cells as a factory — reverse-transcribing its RNA to DNA, integrating it, and mass-producing virus that destroys these cells.
Part (a)
Chemical nature. An antibody is a protein, specifically an immunoglobulin (Ig) — a glycoprotein. Each molecule is Y-shaped and made of four polypeptide chains: two identical light (L) chains and two identical heavy (H) chains held by disulphide bonds, so it is represented as H₂L₂. It has antigen-binding sites at the tips of the arms.
Cells that produce them. Antibodies are secreted by plasma cells, which are the differentiated form of B-lymphocytes (B cells) produced after a B cell is activated by its antigen.
Active vs passive immunity (on the basis of antibodies).
| Feature | Active immunity | Passive immunity |
|---|---|---|
| Antibodies | Made by the person's own body after antigen exposure | Ready-made antibodies given from outside |
| Onset & duration | Slow to appear but long-lasting | Immediate but short-lived |
| Memory | Memory cells formed | No memory |
Showing the 12 most recent of 13 on this concept.
- AP EAPCET 2026Set ap-2026-05-19-FN1 markMCQQ.Which of the following immunoglobulin (Ig) is present in colostrum? (A) Ig A (B) Ig G (C) Ig M (D) Ig E
›Reveal solutionSolution
Colostrum is rich in secretory IgA, giving the newborn passive mucosal immunity.
Answer: (A).
Concept and Intuition
IgA is the major antibody found in body secretions — saliva, tears, mucus, and
milk. During the first few days of lactation, the mammary gland secretes
colostrum which is loaded with IgA (as well as some immune cells). This gives
the newborn's gut passive, ready-made immunity before its own adaptive immune
system fully matures.
Step-by-Step Solution
- Colostrum = first milk secreted post-partum.
- It is especially rich in the antibody class Ig A (secretory form, dimeric, resistant to gut enzymes).
- This passively protects the infant's gastrointestinal tract from pathogens. …
- AP EAPCET 2026Set ap-2026-05-20-FN1 markMCQQ.After infection, HIV attack on these cells (A) Kupfer cells (B) B-cells (C) Dendric cells (D) Mast cells
›Reveal solutionSolution
This tests which immune cells HIV physically infects after it enters the body. Of the
four listed, dendritic cells are the genuine HIV target — the answer is (C).
Concept and Intuition
HIV is a retrovirus that can only enter a cell that displays the right "lock" on its
surface — the CD4 receptor together with a co-receptor (CCR5 on macrophages/dendritic
cells, or CXCR4 on T-cells). Cells lacking this combination are simply invisible to the
virus, no matter how important they are to immunity otherwise.
Step-by-Step Solution
- HIV's envelope glycoprotein gp120 binds CD4 plus a chemokine co-receptor to fuse with and enter a host cell.
- Check each option against this requirement:
- Kupffer cells — resident macrophages of the liver; not a documented site of HIV entry/replication in the human immune-response sense being tested here.
- B-cells — do not display the CD4/co-receptor combination HIV needs; not a direct target (though antibody responses against HIV are impaired indirectly).
- Dendritic cells — express CD4 and CCR5/CXCR4; are among the first cells HIV encounters at mucosal surfaces, and also serve as a reservoir (follicular dendritic cells trap virions in lymph node germinal centres and can transmit them to helper T-cells later). …
- AP EAPCET 2026Set ap-2026-05-20-FN1 markMCQQ.Statement I: Part of antibody that binds with the antigen is called paratope. Statement II: Part of antigen that binds with the antibody is known as epitope. (A) Both statements I and II are true (B) Both statements I and II are false (C) Statement I is true. But II is false (D) Statement I is false. But II is true
›Reveal solutionSolution
Paratope (on the antibody) and epitope (on the antigen) are the two matching "lock and key" binding sites of an antigen-antibody interaction — both statements describing them are correct.
Concept and Intuition
Antigen-antibody binding is highly specific, like a lock and key: the antibody presents a small binding region called the paratope (found in the variable region of its heavy and light chains), and the antigen presents a small immunogenic region called the epitope (or antigenic determinant) that the paratope recognises and binds.
Step-by-Step Solution
- Confirm Statement I: the paratope is indeed defined as the part of the antibody (specifically its variable domain) that makes contact with and binds the antigen. True. …
- AP EAPCET 2026Set ap-2026-05-20-FN1 markMCQQ.Assertion (A): Breast feeding during initial period of infant growth is necessary for bringing up a healthy baby. Reason (R): Colostrum contain several antibodies, especially Ig A type. (A) Both (A) and (R) are true. (R) is correct explanation for (A) (B) Both (A) and (R) are true. (R) is not correct explanation for (A) (C) (A) is true. But (R) is false (D) (A) is false. But (R) is true
›Reveal solutionSolution
Colostrum's high IgA antibody content gives newborns passive immunity, which is exactly why early breastfeeding is emphasised for a healthy start — the Reason correctly explains the Assertion.
Concept and Intuition
A newborn's own immune system is not yet fully mature, so it relies on passively acquired antibodies for early protection against infection. Colostrum — the thick, yellowish first milk produced in the initial days after delivery — is especially rich in immunoglobulins, predominantly secretory IgA, which coats the infant's gut mucosa and helps prevent pathogens from establishing infection.
Step-by-Step Solution
- Confirm the Assertion: paediatric and immunology guidance consistently states that breastfeeding, especially in the initial period, is important for a healthy start to infant life. True.
- Confirm the Reason: colostrum is well documented to be rich in antibodies, with secretory IgA being the predominant and most emphasised immunoglobulin class. True. …
- AP EAPCET 2025Set ap-2025-05-19-FN1 markMCQQ.Based on ICTV system, AIDS virus belongs to the genus (A) Ritro viridae (B) HIV (C) Lentivirus (D) viridae
›Reveal solutionSolution
Under ICTV classification, HIV (the AIDS-causing virus) belongs to the genus Lentivirus within family Retroviridae. Answer: (C).
Concept and Intuition
The International Committee on Taxonomy of Viruses (ICTV) classifies viruses using a hierarchical system (order, family, genus, species) based on features like genome type, replication strategy, and morphology. HIV is a retrovirus (an RNA virus that reverse-transcribes its genome into DNA using reverse transcriptase, then integrates into the host genome). Within the family Retroviridae, HIV is placed in the genus Lentivirus — so named ("lenti-" meaning slow) because of its characteristically long and slow disease course (years may pass between infection and the onset of AIDS symptoms), in contrast to other faster-acting retroviruses.
Step-by-Step Solution
- Recall that AIDS is caused by HIV, a member of family Retroviridae.
- Within Retroviridae, ICTV further subdivides viruses into genera based on pathogenic/replicative characteristics. …
- AP EAPCET 2025Set ap-2025-05-19-FN1 markMCQQ.Part of antigen that binds to the specific part of antibody is called (A) Paratope (B) Isotope (C) Biotope (D) Epitope
›Reveal solutionSolution
The antigen's binding region recognized by an antibody is the epitope; the antibody's own binding site is the paratope. Answer: (D).
Concept and Intuition
Antigen-antibody interactions are highly specific, lock-and-key style recognitions:
- The epitope (also called the antigenic determinant) is the small, specific part of an antigen's surface that is recognized by the immune system — specifically, the part that fits into and binds the antibody's variable region.
- The paratope is the complementary structure on the antibody (located at the tips of the variable regions of the heavy and light chains) that physically binds the epitope.
The question asks specifically about the part of the antigen (not the antibody) that binds — that is the epitope, not the paratope.
Step-by-Step Solution
- Identify what is being asked: the specific region of the antigen that binds to the antibody's specific binding site.
- Recall the immunology term for this antigen-side binding region: 'epitope' or 'antigenic determinant.' …
- AP EAPCET 2024Set ap-2024-05-17-AN1 markMCQQ.Process of technique of detecting HIV in suspected AIDS patients (A) ELISA (B) DNA finger printing (C) r-DNA technology (D) Polymerase Chain Reaction
›Reveal solutionSolution
HIV in suspected AIDS patients is detected by ELISA.
Concept and Intuition
Diagnosis of AIDS is done by detecting antibodies produced against HIV, or the viral antigens, in the patient's blood. The ELISA (Enzyme-Linked Immuno-Sorbent Assay) is the standard, sensitive and inexpensive screening test used for this purpose. Techniques like PCR can detect nucleic acid but the named diagnostic technique for HIV screening is ELISA.
Step-by-Step Solution
- The question asks for the technique to DETECT HIV in suspected AIDS patients.
- DNA fingerprinting (B) identifies individuals, not a virus; r-DNA technology (C) is a gene-manipulation tool; PCR (D) amplifies nucleic acid and is used to detect very low amounts but is not the standard screening diagnostic named here.
- The recognised serological diagnostic test for HIV/AIDS is ELISA.
- Hence option (A).
Common Mistakes …
- AP EAPCET 2024Set ap-2024-05-17-FN1 markMCQQ.Assertion [A] : PCR is used to detect HIV Reason [R] : It is based on principle of antigen – antibody interaction (A) A and R are correct. R is the correct explanation of A (B) A and R are correct. R is not the correct explanation of A (C) A is correct but R is incorrect (D) A is incorrect but R is correct
›Reveal solutionSolution
PCR does detect HIV, but its principle is nucleic-acid amplification, not antigen-antibody interaction (which is the ELISA principle). Answer: (C).
Concept and Intuition
HIV can be detected by two fundamentally different approaches:
- PCR (polymerase chain reaction): amplifies the virus's own nucleic acid (proviral DNA or viral RNA) using repeated cycles of denaturation, primer annealing, and Taq-polymerase-driven extension. This can detect the virus very early, even before the immune system has made antibodies.
- ELISA (antigen-antibody based test): detects HIV antibodies (or antigens) in the blood using antigen-antibody binding, which is what the Reason statement is actually describing.
Step-by-Step Solution
- Assess Assertion A: PCR is indeed a well-established, sensitive method used to detect HIV genetic material, particularly valuable during the early window period. A is TRUE. …
- AP EAPCET 2024Set ap-2024-05-17-FN1 markMCQQ.Identify the incorrect match (A) Interferons – Produced by virus infected cell (B) Interleukins – Produced by erythrocytes (C) Paratope – Antigen binding site in antibody (D) Epitope – Antibody binding site in antigen
›Reveal solutionSolution
Interleukins are cytokines produced by leukocytes (white blood cells), not erythrocytes (red blood cells) — this makes (B) the incorrect match among otherwise valid immunology definitions.
Concept and Intuition
This question checks recall of key immunology terms:
- Interferons — proteins produced by virus-infected cells that protect neighbouring uninfected cells from viral spread by inducing an antiviral state; (A) is correct.
- Interleukins — a class of cytokines produced predominantly by leukocytes (white blood cells such as macrophages and T-helper lymphocytes) to communicate between immune cells; erythrocytes (red blood cells), being mature and anucleate in mammals, cannot synthesize such signaling proteins. So (B) is the incorrect match.
- Paratope — the specific region on an antibody's variable domain that binds the antigen; (C) is correct.
- Epitope — the specific region on an antigen that is recognized and bound by an antibody (or immune receptor); (D) is correct (the phrase "antibody binding site in antigen" is a valid alternate description of epitope).
Step-by-Step Solution
- Verify (A): Interferons are indeed produced by virus-infected cells. Correct match.
- Verify (B): Interleukins are produced by leukocytes, not erythrocytes — erythrocytes are non-nucleated in mammals and cannot produce cytokines. Incorrect match — this is the answer. …
- AP EAPCET 2023Set ap-2023-05-22-FN1 markMCQQ.The presence of HIV in the blood of infected person cannot be detected during (A) Window period (B) Incubation period (C) Prepatent period (D) Refractory period
›Reveal solutionSolution
Tests the specific immunological term for the diagnostic blind-spot after HIV infection. Answer: (A) Window period.
Concept and Intuition
Standard HIV tests typically detect antibodies (or antigen/antibody combinations) that the immune system produces in response to infection. It takes time — days to weeks — for these markers to rise to detectable levels. This gap between actual infection and detectability is specifically called the "window period," during which an infected individual can unknowingly test negative and transmit the virus to others.
Step-by-Step Solution
- Recognize the terms: "incubation period" is the time between infection and appearance of symptoms/disease, not test detectability. …
- AP EAPCET 2022Set ap-2022-07-12-AN1 markMCQQ.Identify the monomer forms of the Antibodies among the following groups (A) Ig A, Ig D, Ig E (B) Ig G, Ig A, Ig M (C) Ig G, Ig A, Ig E (D) Ig D, Ig E, Ig G
›Reveal solutionSolution
IgG, IgD, and IgE exist only as monomers, unlike IgM (pentamer) and secretory IgA (dimer) — so the "monomer forms" group is IgD, IgE, IgG.
Concept and Intuition
Antibody structure is built from a basic Y-shaped monomer unit (two heavy chains + two light chains), but different immunoglobulin classes can assemble into higher-order multimers via a joining (J) chain:
- IgM is secreted as a pentamer (five monomer units joined by a J chain), giving it high avidity — ideal as the first-responder antibody in a primary immune response.
- IgA commonly exists as a dimer in secretions (secretory IgA, joined by a J chain and secretory component), though monomeric IgA also circulates in serum.
- IgG, IgD, and IgE exist solely as single monomeric units — they never assemble into dimers or pentamers.
Step-by-Step Solution
- Recall each immunoglobulin's typical structural form: IgG (monomer), IgA (monomer/dimer), IgM (pentamer), IgD (monomer), IgE (monomer).
- Eliminate option (B), which includes IgM — clearly not "monomer" (it's pentameric). …
- AP EAPCET 2022Set ap-2022-07-12-FN1 markMCQQ.Colostrum has abundant antibodies of type (A) Ig A (B) Ig G (C) Ig E (D) Ig K
›Reveal solutionSolution
Colostrum is particularly rich in secretory IgA antibodies, providing passive mucosal immunity to the newborn.
Concept and Intuition
Colostrum differs from mature breast milk in being especially concentrated in immunoglobulins, predominantly secretory IgA, which is adapted to survive in the infant's gut and line the mucosal surfaces, protecting against enteric pathogens before the infant's own immune system matures.
Step-by-Step Solution
- Recall that IgA is the principal antibody class found in mucosal secretions (saliva, tears, breast milk, gut lining).
- Colostrum specifically has very high concentrations of secretory IgA compared to other antibody classes. …
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