Q.(a) Give an account of the generalised structure of an antibody molecule produced by B-lymphocytes in response to the pathogen.
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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 — STD Prevention Methods
STD Prevention Methods — A First Look
Think of your body like a house. You lock the doors and windows to keep out unwanted visitors. Sexually Transmitted Diseases (STDs) are infections that can enter the body through sexual contact — and prevention is about building those "locks" before any risk arises.
What Are STDs and Why Prevention Matters
STDs (also called STIs — Sexually Transmitted Infections) are infections passed from one person to another during sexual activity. They include diseases like HIV, syphilis, gonorrhoea, herpes, and hepatitis B. Some can be cured with treatment; others, like HIV, cannot be cured but can be managed. The key point: prevention is far better than cure, because many STDs can cause long-term health problems — infertility, organ damage, or even death — and some can be passed from a mother to her baby during pregnancy or childbirth.
The NCERT textbook emphasises that prevention is the most effective strategy against STDs. Once infected, some diseases have no complete cure, and treatment can be expensive, painful, or lifelong.
The Core Prevention Methods
The NCERT approach groups prevention into three broad categories: behavioural, mechanical, and medical. Let's look at each.
1. Behavioural Prevention — The First Line of Defence
This is about choices you make before any sexual contact occurs. It is the simplest and most reliable method.
- Abstinence: Not having sexual intercourse at all. This is the only 100% guaranteed way to avoid STDs. For young people, delaying sexual activity is strongly recommended.
- Mutual monogamy: Having a sexual relationship with only one partner, who is also faithful to you, and both of you have been tested and are free of STDs.
- Avoiding high-risk behaviour: Not having multiple partners, not using alcohol or drugs before sex (which can impair judgment), and avoiding sex with people whose health status you do not know.
Behavioural methods require awareness and self-discipline. They cost nothing but demand personal responsibility.
2. Mechanical Prevention — Physical Barriers
These are devices that physically block the transfer of body fluids (semen, vaginal fluid, blood) between partners.
- Condoms (male and female): A thin latex or polyurethane sheath worn over the penis (male condom) or inside the vagina (female condom). When used correctly every time, condoms are highly effective against most STDs, including HIV.
- Dental dams: A square sheet of latex used during oral sex to prevent direct contact.
Condoms are not 100% effective — they can break or be used incorrectly. Also, they do not protect against infections spread by skin-to-skin contact in areas not covered by the condom (like herpes or genital warts). But they remain the best available barrier method.
3. Medical Prevention — Vaccines and Treatment
- Vaccination: Safe and effective vaccines exist for Hepatitis B and Human Papillomavirus (HPV) — which causes cervical cancer and genital warts. The NCERT recommends that adolescents receive these vaccines before becoming sexually active.
- Regular testing and early treatment: If you are sexually active, getting tested regularly (even without symptoms) helps catch infections early. Many STDs have no visible symptoms. Early treatment can cure some infections and prevent complications. …
Part (a)
An antibody (immunoglobulin) is a Y-shaped protein made of four polypeptide chains — two identical long heavy (H) chains and two identical short light (L) chains — held together by disulphide bonds. Hence it is represented as H2L2. Each chain has a variable region (forms the antigen-binding site at the tips of the arms) and a constant region. …
Part (a): An antibody is a Y-shaped H2L2 protein of two heavy and two light chains linked by disulphide bonds, with variable regions that bind antigen and constant regions.
Part (b): NACO, WHO and NGOs prevent HIV spread by screening blood, using disposable needles, distributing free condoms, and providing antiretroviral therapy.
Part (a)
When a pathogen (antigen) enters the body, B-lymphocytes produce proteins called antibodies (immunoglobulins) that specifically recognise it. The generalised structure:
- Each antibody is Y-shaped and consists of four polypeptide chains.
- Two of these are longer, called heavy (H) chains; two are shorter, called light (L) chains. The two heavy chains are identical and the two light chains are identical, so an antibody is written as H2L2.
- The chains are held together by disulphide (-S-S-) bonds.
- Each chain has a variable region (V), and the variable regions of the H and L chains at the tips of the two arms form the two identical antigen-binding sites; the rest is the constant region (C), which determines the antibody's class and effector functions. …
Showing the 12 most recent of 21 on this concept.
- CBSE 2026Set V11 markMCQQ.Sexually transmitted infection that can be completely curable is(a) Gonorrhea(b) Hepatitis-B(c) AIDS(d) Genital herpes
›Reveal solutionSolution
Gonorrhea is bacterial and fully curable; the other three are viral and not completely curable.
Gonorrhea is caused by the bacterium Neisseria gonorrhoeae and, if detected early, is completely curable with antibiotics. Hepatitis-B, AID …
- CBSE 2026Set ANNUAL1 markMCQQ.Which of the following is not a sexually transmitted disease?(a) Gonorrhoea(b) Herpes(c) Syphilis(d) Tuberculosis
›Reveal solutionSolution
Gonorrhoea, herpes and syphilis are classic sexually transmitted diseases (STDs); tuberculosis is a respiratory bacterial infection, not an STD.
Sexually transmitted diseases (also called venereal diseases or RTIs) spread through sexual contact and include gonorrhoea and syphilis (bacterial), genital herpes (viral), as well as HIV/AIDS, genital warts and trichomoniasis. Tuberculosi …
- CBSE 2026Set ANNUAL1 markMCQQ.Identify the parts labelled A, B, C and D in the diagram of an antibody molecule and select the correct options :(a) A – Light chains, B – Heavy chains, C – Antigen-binding sites, D – Disulphide bonds(b) A – Disulphide bonds, B – Antigen-binding sites, C – Heavy chains, D – Light chains(c) A – Antigen-binding sites, B – Light chains, C – Heavy chains, D – Disulphide bonds(d) A – Antigen-binding sites, B – Disulphide bonds, C – Light chains, D – Heavy chains
›Reveal solutionSolution
An antibody is a Y-shaped H2L2 molecule: the two arm tips are the antigen-binding sites, the two short outer chains are light chains, the two long chains are heavy chains, and the chains are held together by disulphide bonds — matching option (c).
In the CBSE/NCERT Human Health and Disease chapter, an antibody (immunoglobulin) is written as H2L2:
- A – Antigen-binding sites: the two tips of the Y (the variable regions) where the specific antigen binds.
- B – Light chains: two short polypeptide chains.
- C – Heavy chains: two long polypeptide chains. …
- CBSE 2026Set SEM31 markMCQQ.________ is not a veneral disease.(a) Hepatitis-B(b) Malaria(c) Trichomoniasis(d) Syphilis
›Reveal solutionSolution
Hepatitis-B, Trichomoniasis and Syphilis are sexually transmitted (venereal) diseases; Malaria is transmitted by the Anopheles mosquito, so it is NOT a venereal disease.
Venereal / Sexually Transmitted Diseases (STDs) are transmitted through sexual contact — examples include syphilis (bacterial), trichomoniasis (protozoan), and hepatitis-B (viral, also blood-borne but STD-classified). Malaria is caused by Plasmodium and spreads through the bite of the female Anopheles mosquito, i.e. it is a …
- CBSE 2025Set F1 markMCQQ.Which of the following is a sexually transmitted disease (STD)?(a) Syphilis(b) Gonorrhoea(c) Both (A) and (B)(d) None of these
›Reveal solutionSolution
Both syphilis and gonorrhoea are sexually transmitted diseases, so the answer is Both (A) and (B).
Sexually transmitted diseases (STDs / reproductive tract infections) spread mainly through sexual contact. Bacterial STDs include:
- Syphilis (caused by Treponema pallidum),
- Gonorrhoea (caused by Neisseria gonorrhoeae). …
- CBSE 2025Set ANNUAL1 markQ.Fill in the blank: Antibody molecule has ___ peptide chains.
›Reveal solutionSolution
An antibody (immunoglobulin) molecule is a Y-shaped protein made of four polypeptide chains: two identical heavy chains and two identical light chains.
Each antibody unit consists of two long, identical heavy (H) chains and two short, identical light (L) chains, held together by disulfide bonds to form a characteristic Y-shaped structure. Each chain has a variable region (which determines antigen spec …
- CBSE 2024Set D1 markQ.Write True or False: Syphilis is a sexually transmitted disease.
›Reveal solutionSolution
The statement is True — syphilis is indeed a sexually transmitted disease, caused by the bacterium Treponema pallidum.
Sexually transmitted diseases (STDs), also called reproductive tract infections (RTIs) or venereal diseases, are infections or diseases transmitted through sexual contact. Common examples listed in the NCERT syllabus include gonorrhoea, syphilis, genital herpes, chlamydiasis, genital warts, trichomoniasis, hepatitis-B, and HIV/AIDS.
…
- CBSE 2024Set ANNUAL1 markMCQQ.Which one of the following diseases can be transmitted through sexual contact?(a) COVID 19(b) Typhoid(c) Hepatitis B(d) Cholera
›Reveal solutionSolution
Among the given options, Hepatitis B is a sexually transmitted disease (STD/RTI) - it also spreads through blood and body fluids.
Sexually Transmitted Diseases (STDs), also called Reproductive Tract Infections (RTIs), spread through sexual contact. Common examples include gonorrhoea, syphilis, genital herpes, chlamydiasis, genital warts, trichomoniasis, Hepatitis B, and HIV/AIDS.
- Hepatitis B virus spreads through unprotected sexual contact, contaminated needles/blood transfusion, and from mother to child. …
- CBSE 2023Set ANNUAL1 markMCQQ.Not an example of sexually transmitted disease is -(a) Gonorrhea(b) Syphilis(c) Chlamydiasis(d) Cancer
›Reveal solutionSolution
Gonorrhea, syphilis and chlamydiasis are bacterial STDs transmitted through sexual intercourse; cancer is not an infectious disease and is not classified as an STD.
Sexually transmitted diseases (STDs), also called venereal diseases (VD) or reproductive tract infections (RTI), are infections transmitted through sexual contact. Common examples include gonorrhea, syphilis, genital herpes, chlamydiasis, genital warts, trichomoniasis, hepatitis-B and HIV/AIDS.
…
- CBSE 2023Set ANNUAL1 markMCQQ.Genital warts is an STD spread by which?(a) hepatitis A(b) herpes virus(c) papilloma virus(d) Trichomonas
›Reveal solutionSolution
Genital warts are caused by human papilloma virus (HPV).
Genital warts are a sexually transmitted disease (STD) caused by the human papilloma virus (HPV). Certain HPV types also predispose to cervical cancer, which is why HPV vaccination is recommended. Hepatitis-A is not an STD in this sense, herpes virus causes genital he …
- CBSE 2023Set ANNUAL1 markMCQQ.Antibodies in the blood are secreted by which?(a) monocytes(b) neutrophils(c) lymphocytes(d) basophils
›Reveal solutionSolution
Lymphocytes (B-cells/plasma cells) secrete antibodies.
Antibodies (immunoglobulins) are the proteins of humoral immunity. They are produced by B-lymphocytes, which on activation differentiate into plasma cells that secrete large amounts of antibody into the blood. Monocytes, neutrophils and basoph …
- CBSE 2022Set ANNUAL1 markMCQQ.Genital warts spread through which of the following?(a) hepatitis-A(b) herpes virus(c) Trichomonas(d) papilloma virus
›Reveal solutionSolution
Genital warts are caused by the human papilloma virus (HPV), so the answer is (D).
Genital warts are a sexually transmitted infection (STI) caused by certain types of the human papilloma virus (HPV). The infection spreads through sexual contact and causes wart-like growths in the genital region; some HPV types are also linked to cervical cancer.
…
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