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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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 25 on this concept.
- CBSE 2026Set V11 markQ.Human immuno deficiency virus replicates in ___________.
›Reveal solutionSolution
HIV first enters and replicates within macrophages, so the word-bank answer for 'Human immunodeficiency virus replicates in ___' is macrophages.
After entering the body, the human immunodeficiency virus (HIV) enters macrophages. Inside a macrophage its single-stranded RNA genome is reverse-transcribed into viral DNA (by the enzyme reverse transcriptase); this viral DNA is incorporated into the host cell's DNA and directs the production of new virus particles. The infected macrophage keeps releasing virus and in this sense behaves as an HIV 'factory'. The virus then also enters and multipl …
- CBSE 2026Set A1 markMCQQ.Which of the following viruses has reverse transcriptase enzyme?(a) TMV(b) Bacteriophage(c) HIV(d) All of these
›Reveal solutionSolution
HIV is a retrovirus and carries reverse transcriptase; the correct option is (c).
Reverse transcriptase is the enzyme that synthesises DNA using an RNA template — the reverse of normal transcription. It is characteristic of retroviruses.
…
- CBSE 2026Set ZOOLOGY1 markMCQQ.Which is the genetic material of HIV?(a) Single-stranded RNA(b) Single-stranded DNA(c) Double-stranded DNA(d) Double-stranded RNA
›Reveal solutionSolution
HIV (Human Immunodeficiency Virus) is a retrovirus carrying single-stranded RNA as its genome.
HIV belongs to the retrovirus group, meaning its genetic material is RNA, not DNA. Specifically, HIV carries two copies of single-stranded (+)sense RNA enclosed within a protein capsid. On infecting a helper T-lymphocyte (CD4+ cell), the viral enzyme reverse transcriptase uses this RNA as a template to synthesize viral DNA, which then integrates into the host cell's genome as a provirus. This DNA subsequently directs the pro …
- CBSE 2026Set ANNUAL1 markMCQQ.Test for AIDS is :(a) Widal test(b) Biopsy test(c) ELISA test(d) CRY organism test
›Reveal solutionSolution
AIDS (caused by HIV) is diagnosed using the ELISA (Enzyme-Linked Immunosorbent Assay) test.
ELISA (Enzyme-Linked Immunosorbent Assay) detects the presence of HIV antigens or the antibodies produced by the body's immune system against HIV in a patient's blood serum. It is the standard screening test for AIDS. (Widal test is for typhoid; biopsy is for examining tissue, e.g. for c …
- 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 2025Set ANNUAL1 markMCQQ.The type of HIV virus is -(a) Adenovirus(b) Bacteriophage virus(c) Retrovirus(d) Rotavirus
›Reveal solutionSolution
HIV carries its genome as RNA and uses reverse transcriptase to make a DNA copy inside the host cell, the defining feature of a retrovirus.
HIV (Human Immunodeficiency Virus) belongs to the retrovirus group: its genetic material is single-stranded RNA, and on infecting a host lymphocyte (helper T-cell) it uses the enzyme reverse transcriptase to synthesise viral DNA from the RNA template, which then integrates into the host genome. This 'reverse' flow of information ( …
- 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 ANNUAL1 markQ.[Case/Source-based passage] The word AIDS stands for Acquired Immuno Deficiency Syndrome. AIDS was first reported in 1981. AIDS mainly caused by the HIV (Human Immunodeficiency Virus). It is also called retrovirus. HIV infection mainly occurs by sexual contact with infected person. It is transmitted from the infected mother to her child through the Placenta. AIDS has no cure, but prevention is the best care. In our country, the National AIDS Control Organisation (NACO) and in states, the State AIDS Control Organisations (SACO) are making people aware towards AIDS education. World Health Organisation (WHO) has started a number of programmes to prevent the spreading of HIV infection. For this, we have need to aware and alert, so that India could lead forward in the direction of Human Welfare. On 1st day of December, AIDS day is celebrated. Read the above passage carefully and answer the questions given below:(a) How AIDS spreads from one infected person to others?
›Reveal solutionSolution
AIDS spreads mainly through sexual contact with an infected person and from an infected mother to her child via the placenta.
As stated in the passage, HIV infection mainly spreads through sexual contact with an infected person. It can also be transmitted from an infected mother to her child through the placenta during pregnancy. Beyond the passage, HIV can also spread through transfusion of contaminated blood/blood products, and through sharing of infected/contaminated needles an …
- CBSE 2024Set ANNUAL1 markQ.(Same passage as Q26.)(b) "Prevention is the best cure" - Clarify in context to AIDS.
›Reveal solutionSolution
Since AIDS has no cure, avoiding exposure to HIV through preventive practices is the only effective way to protect against it.
As the passage states, 'AIDS has no cure, but prevention is the best care.' Since there is currently no drug or vaccine that can cure or fully eliminate HIV infection from the body, the only truly effective strategy against AIDS is to prevent oneself from getting infected in the first place — for example, by avoiding unprotected/casual sexual contact, using precautions such as condoms, ensuring blood used for transfusion is screened, and avoiding the sharing of needles/syringes. This is why awareness and safe practices, rather than treatment aft …
- CBSE 2024Set E1 markMCQQ.HIV attacks on which of the following cells?(a) B-cells(b) T-cells(c) Epithelial cell(d) T-helper cell
›Reveal solutionSolution
HIV attacks the helper T-lymphocytes (CD4+ cells), destroying immunity and causing AIDS.
After entering the body, HIV enters macrophages and then attacks the helper T-lymphocytes (T-helper cells, which carry CD4 receptors). Inside them the viral RNA is reverse-transcribed to DNA, integrated into the host genome, and made to produce more virus particles. The progressive destruction of helper T-cells cripples the immune system, …
- CBSE 2023Set ANNUAL1 markMCQQ.Retrovirus is the pathogen of which of the following diseases?(a) filariasis(b) syphilis(c) AIDS(d) both(b) and (c)
›Reveal solutionSolution
A retrovirus (HIV) causes AIDS.
Retroviruses are RNA viruses that carry the enzyme reverse transcriptase, which makes a DNA copy of their RNA genome inside the host cell. The Human Immunodeficiency Virus (HIV), the cause of AIDS, is such a retrovirus; it infects and destroys helper T-cells, weakening immunit …
- 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 …
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