BiologyISC Class 12 · Human Health and Diseases

Immunity, Antibodies and Lymphoid Organs: ISC Class 12 Biology

Updated 15 October 2026 · Fact-checked

Immunity is the body's ability to fight disease-causing organisms. Innate immunity is non-specific, present from birth, and uses physical, physiological, cellular and cytokine barriers. Acquired immunity is specific and has memory: B cells make Y-shaped H₂L₂ antibodies (IgG, IgA, IgM, IgD, IgE), T cells give cell-mediated immunity. Lymphocytes mature in bone marrow and thymus.

Notice the H₂L₂ design: two heavy and two light chains joined by disulphide bonds, with the variable tips binding the antigen.

Illustration drawn for QuestionBuddy.

Understand Immunity, Antibodies and Lymphoid Organs

Immunity is the overall ability of the host to fight disease-causing organisms, conferred by the immune system.

1. Innate immunity: non-specific, present at birth, by four barriers.

Barrier	Examples
Physical	skin (main barrier); mucus of respiratory, digestive and urogenital tracts traps microbes
Physiological	acid in the stomach, saliva in the mouth, tears from the eyes
Cellular	PMNL (neutrophils), monocytes, natural killer cells in blood; macrophages in tissues: phagocytose and destroy microbes
Cytokine	interferons secreted by virus-infected cells protect non-infected cells from further viral infection

Interferons (definition, source, function). Small proteins (glycoproteins) secreted by virus-infected cells; they diffuse to neighbouring cells and make them resistant to viral replication, and they activate NK cells and macrophages. Their production indicates a viral infection. α-interferon is used in immunotherapy of cancer.

2. Acquired immunity: pathogen-specific with memory. The first encounter gives a low-intensity primary response; a later encounter with the same pathogen gives a rapid, intense secondary (anamnestic) response. Carried out by B and T lymphocytes.

Humoral (antibody-mediated) immunity: B lymphocytes produce antibodies into blood and lymph. T cells do not secrete antibodies but help B cells to make them.
Cell-mediated immunity (CMI): T lymphocytes act directly; CMI is responsible for graft rejection, so tissue and blood-group matching and lifelong immunosuppressants are needed after transplants. The body distinguishes 'self' from 'non-self'.

Structure of an antibody (draw-and-label). A Y-shaped molecule of four polypeptide chains: two long heavy chains and two short light chains, written H₂L₂, held together by disulphide bonds (–S–S–). Each chain has a variable region at the tip of the arms, forming the two antigen-binding sites, and a constant region. A flexible hinge joins the arms to the stem. Labels: heavy chain, light chain, variable region, constant region, antigen-binding site (×2), disulphide bonds, hinge.

Five classes of antibodies (immunoglobulins).

Class	Occurrence	Function
IgG	most abundant in serum	crosses the placenta, giving the foetus passive immunity; main antibody of the secondary response
IgA	colostrum, saliva, tears, mucus	protects mucous surfaces; gives the newborn passive immunity through mother's milk
IgM	serum; largest (a pentamer)	first antibody made in a primary response
IgD	on the surface of B cells	acts as an antigen receptor that activates B cells
IgE	bound to mast cells and basophils	mediates allergy; defence against parasites

Active and passive immunity.

Active: the host makes its own antibodies after exposure to antigens (natural infection or vaccine). Slow to develop; long-lasting.
Passive: ready-made antibodies are given. Immediate; short-lived. Examples: IgA in colostrum, IgG across the placenta, antitoxin for tetanus, anti-venom for snakebite (passive immunisation).

Vaccination and immunisation. Based on memory: a vaccine (inactivated or weakened pathogen, or its antigenic proteins) triggers a primary response and memory B and T cells; on real infection a massive secondary response neutralises the pathogen. Recombinant DNA technology produces antigenic polypeptides in bacteria or yeast (hepatitis B vaccine from yeast).

Allergy and autoimmunity. Allergy: an exaggerated immune response to environmental antigens (allergens: dust mites, pollen, animal dander); IgE antibodies cause mast cells to release histamine and serotonin; sneezing, watery eyes, running nose, difficulty in breathing, rashes; treated with anti-histamines, adrenaline and steroids. Autoimmunity: the body attacks its own cells (rheumatoid arthritis).

Lymphoid organs. Organs where origin and/or maturation and proliferation of lymphocytes occur.

	Primary lymphoid organs	Secondary lymphoid organs
Examples	bone marrow, thymus	spleen, lymph nodes, tonsils, Peyer's patches of small intestine, appendix; MALT
Role	immature lymphocytes differentiate into antigen-sensitive lymphocytes	lymphocytes meet antigens and proliferate into effector cells

Bone marrow: main lymphoid organ; all blood cells, including lymphocytes, are produced here.
Thymus: lobed organ near the heart beneath the breastbone; large at birth, shrinks to a very small size by puberty. Together with the bone marrow, it supplies the surroundings in which T lymphocytes develop and mature.
Spleen: large, bean-shaped; lymphocytes and phagocytes; filters blood by trapping blood-borne microbes; large reservoir of erythrocytes.
Lymph nodes: small solid structures along the lymphatic system; trap microbes and antigens in lymph and tissue fluid; activate lymphocytes there.
MALT (mucosa-associated lymphoid tissue) in the lining of the respiratory, digestive and urogenital tracts: about 50 per cent of the body's lymphoid tissue.

Immunity, Antibodies and Lymphoid Organs: key facts

Antibody formula: Chains two heavy and two light (H₂L₂)
Bonds disulphide (–S–S–)
Antigen-binding sites two, at the variable regions of the arms
Shape Y, with a flexible hinge; Two heavy and two light chains, joined by disulphide bonds.
Response with memory: 1.First exposure: primary response, slow and weak, IgM first
2.memory B and T cells formed
3.Second exposure: secondary response, fast and strong; Basis of vaccination.
Passive examples: Colostrum IgA to the newborn
Placenta IgG to the foetus
Antitoxin tetanus
Anti-venom snakebite; Ready-made antibodies; quick but short-lived.
Lymphoid organs: Primary bone marrow, thymus; lymphocytes form and mature
Secondary spleen, lymph nodes, tonsils, Peyer's patches, appendix, MALT; lymphocytes meet antigens and proliferate
MALT about 50% of lymphoid tissue; Maturation versus meeting antigens.

Chains	two heavy and two light (H₂L₂)
Bonds	disulphide (–S–S–)
Antigen-binding sites	two, at the variable regions of the arms
Shape	Y, with a flexible hinge

Colostrum	IgA to the newborn
Placenta	IgG to the foetus
Antitoxin	tetanus
Anti-venom	snakebite

Primary	bone marrow, thymus; lymphocytes form and mature
Secondary	spleen, lymph nodes, tonsils, Peyer's patches, appendix, MALT; lymphocytes meet antigens and proliferate
MALT	about 50% of lymphoid tissue

How to answer Immunity, Antibodies and Lymphoid Organs questions

ISC sets one-mark identifications (IgE in allergy, interferons with viral disease, lymphoid organ assertion-reason), antibody diagrams, active versus passive and innate versus acquired differences, and vaccine reasoning.

1Read the clue and name one answer: allergy → IgE; colostrum → IgA; placenta → IgG; first in primary response → IgM; B-cell receptor → IgD.
2For an antibody diagram, draw a Y with two long and two short chains and at least six labels.
3For differences, use parallel rows: specificity, memory, time to act, duration, examples.
4Primary lymphoid organs are where lymphocytes form and mature; secondary are where they meet antigen and multiply.
5In vaccine questions, explain memory B and T cells and the faster secondary response.
6For passive immunisation, say ready-made antibodies are needed when the pathogen or toxin acts too fast for the body to respond.

Immunity, Antibodies and Lymphoid Organs in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Active and passive immunisation after an injury

Suppose a farmer who has never been vaccinated against tetanus cuts his foot on a rusty plough, and the doctor gives him two injections: an antitoxin now and the first dose of tetanus vaccine.

Explain why both injections are given.

Try it first, then show the solutionSolution

Tetanus toxin acts quickly; the farmer has no antibodies, and making his own would take too long.
The antitoxin contains ready-made antibodies that neutralise the toxin at once: passive immunity, immediate but short-lived.
The vaccine triggers his own primary response and memory B and T cells: active immunity, slow to start but long-lasting.
Future exposure will then meet a fast secondary response.

Answer: Antitoxin gives immediate passive protection; the vaccine builds lasting active immunity with memory.

Memory hook: Borrowed now, built for later.

Tricks to remember Immunity, Antibodies and Lymphoid Organs

Short memory aids. Say each one aloud once, then use it on a question the same day.

GAMDE in one line each
G goes across the placenta; A in milk and tears; M is the Monster (pentamer) that comes first; D sits on B cells; E for allErgy.
Use it when: Matching antibody classes to clues.
B makes, T kills
B cells make antibodies (humoral); T cells act directly (cell-mediated, graft rejection).
Use it when: Humoral versus CMI.
Primary = Born and Trained
Bone marrow and Thymus are where lymphocytes are born and trained; secondary organs are the battlefields.
Use it when: Primary versus secondary lymphoid organs.

Immunity, Antibodies and Lymphoid Organs: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Antibody structure · 3 marks · Short answerBoard level

Draw a labelled diagram of an antibody molecule. Why is it written as H₂L₂?

Try it first, then show the solutionSolution

Y-shaped molecule with two long heavy chains and two short light chains, each arm tipped with a variable region forming an antigen-binding site (1 mark).
Labels: heavy chain, light chain, variable region, constant region, antigen-binding site, disulphide bonds, hinge (1 mark).
It has two heavy (H) and two light (L) polypeptide chains, so H₂L₂ (1 mark).

Answer: Four chains: two heavy, two light (H₂L₂), with two antigen-binding sites.

Example 2: Innate barriers · 2 marks · Short answerBoard level

Name the four types of barriers of innate immunity, giving one example of each.

Try it first, then show the solutionSolution

Physical: skin or mucus; physiological: stomach acid, saliva or tears (1 mark).
Cellular: neutrophils, monocytes, NK cells or macrophages; cytokine: interferons (1 mark).

Answer: Physical, physiological, cellular, cytokine (examples as above).

Example 3: Spleen assertion-reason · 2 marks · Short answerHarder board level

Assertion: The spleen traps micro-organisms carried in the blood. Reason: The spleen is a primary lymphoid organ in which immature lymphocytes become antigen-sensitive. Evaluate both statements and correct any error.

Try it first, then show the solutionSolution

The assertion is true: the spleen contains lymphocytes and phagocytes and filters blood by trapping blood-borne microbes (1 mark).
The reason is false: the spleen is a secondary lymphoid organ, where lymphocytes meet antigens and proliferate; the primary lymphoid organs, where lymphocytes mature, are the bone marrow and thymus (1 mark).

Answer: Assertion true, reason false; the spleen is a secondary lymphoid organ.

Example 4: Colostrum and placenta · 3 marks · Short answerHarder board level

Why is breastfeeding during the first days after birth recommended? Name the antibody involved and another antibody the baby received before birth. What type of immunity is this?

Try it first, then show the solutionSolution

The yellowish colostrum secreted in the first days is rich in IgA antibodies that protect the infant's gut and mucous surfaces (1 mark).
IgG crossed the placenta before birth (1 mark).
Both are ready-made antibodies from the mother: natural passive immunity, which is short-lived (1 mark).

Answer: Colostrum IgA; placental IgG; passive immunity.

Example 5: Primary and secondary response · 3 marks · Short answerHarder board level

A person is given the first dose of a vaccine, and some months later the same antigen enters the body. Compare the two responses and explain the difference.

Try it first, then show the solutionSolution

First exposure: primary response, slow and of low intensity (1 mark).
Second exposure: secondary (anamnestic) response, rapid and much stronger, with far more antibodies (1 mark).
The first exposure left memory B and T cells that recognise the antigen at once and multiply quickly (1 mark).

Answer: Secondary response is faster and stronger because of memory cells.

Example 6: Organ transplant and immunity · 4 marks · Short answerHOTS / competency

Suppose a patient receives a kidney from an unrelated donor. (a) Which arm of immunity may reject it? (b) Why are tissue and blood-group matching done? (c) Why must the patient take immunosuppressants, and what risk does this bring?

Try it first, then show the solutionSolution

(a) Cell-mediated immunity by T lymphocytes, which recognise the graft as non-self (1 mark).
(b) The closer the match of tissue antigens and blood group, the less foreign the graft appears and the weaker the rejection (1 mark).
(c) Even after matching, T cells still respond; immunosuppressants damp the immune response (1 mark).
The patient then fights infections poorly and needs protection from pathogens (1 mark).

Answer: (a) CMI. (b) To reduce foreignness. (c) To prevent rejection, at the cost of higher infection risk.

Where students lose marks in Immunity, Antibodies and Lymphoid Organs

The slips examiners see most often, and what to write instead.

Mistake: Listing all five antibodies when one is asked
In 2024 examiners reported that candidates listed all five immunoglobulins instead of naming IgE for allergic reactions.
Do this: Learn one clue per class and give one answer.
Usually costs: 1 mark.
Mistake: Not linking interferons to viral disease
Examiners (2024) found distractors chosen instead of the viral disease when interferon production was the clue.
Do this: Interferons are made by virus-infected cells.
Usually costs: 1 mark.
Mistake: Confusing primary and secondary lymphoid organs
Examiners (2024) noted the wrong option in the lymphoid-organ assertion-reason because candidates mixed up the two groups.
Do this: Primary: bone marrow, thymus only.
Usually costs: 1 mark.
Mistake: Naming a fungal infection for allergic rashes
In 2025 examiners reported fungal or ringworm infection for rashes after a hair dye, or only symptoms without the condition.
Do this: A rapid reaction after contact with a chemical points to allergy (hypersensitivity).
Usually costs: 1 mark.

Exam tips

Write H₂L₂ with subscripts and name the bonds as disulphide bonds.
For passive immunity, always say 'ready-made' or 'preformed' antibodies.
Give MALT's share (about 50%) when asked about lymphoid tissue.
Graft rejection is cell-mediated, not antibody-mediated, in ISC answers.
Use 'anamnestic' only alongside 'secondary response'.

Practice questions on Immunity, Antibodies and Lymphoid Organs

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The antibody responsible for allergic reactions is:
(a) IgA (b) IgG (c) IgM (d) IgE
Hint
Mast cells.
Answer
(d) IgE.
Q2 · 2 marks · Short answerBoard level
Distinguish between active and passive immunity (two points) with one example of each.
Hint
Who makes the antibodies?
Answer
Active: host makes its own antibodies; slow to develop, long-lasting; e.g. after vaccination or infection. Passive: ready-made antibodies given; immediate, short-lived; e.g. antitoxin for tetanus or IgA in colostrum.
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The thymus is large at birth and becomes very small by puberty.
Reason (R): The thymus is a secondary lymphoid organ where lymphocytes meet antigens.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Primary or secondary?
Answer
(c). The thymus is a primary lymphoid organ.
Q4 · 2 marks · Short answerHarder board level
What are interferons? Give their source and function, and name the type of disease they indicate.
Hint
Cytokine barrier.
Answer
Small proteins (glycoproteins) secreted by virus-infected cells; they protect neighbouring non-infected cells from viral replication and activate NK cells and macrophages. Their production indicates a viral infection.
Q5 · 2 marks · Short answerHarder board level
State two functions of the spleen and explain the role of lymph nodes.
Hint
Filter.
Answer
Spleen filters blood by trapping blood-borne microbes and acts as a reservoir of erythrocytes. Lymph nodes trap microbes and antigens in lymph and tissue fluid; the trapped antigens activate lymphocytes there, starting an immune response.
Q6 · 3 marks · Short answerHarder board level
Explain the cause and symptoms of allergy, naming two chemicals involved and two drugs used.
Hint
IgE and mast cells.
Answer
Allergy is an exaggerated immune response to allergens (dust mites, pollen, animal dander). IgE antibodies are produced and mast cells release histamine and serotonin, causing sneezing, watery eyes, running nose, difficulty in breathing. Drugs: anti-histamines, adrenaline, steroids (any two).
Q7 · 3 marks · Short answerHOTS / competency
Suppose a child is born without a thymus. Predict which arm of immunity is most affected and give two consequences.
Hint
T-cell maturation.
Answer
T lymphocytes cannot mature properly, so cell-mediated immunity is most affected. Consequences: poor defence against viruses and intracellular pathogens (frequent infections); weaker antibody responses too, because helper T cells are needed to help B cells; grafts may not be rejected.
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose a girl uses a new face cream and within hours develops red, itchy rashes on her face. The doctor finds raised levels of one antibody class and gives an anti-histamine.
(a) Name the condition. (1)
(b) Which antibody class is raised and which cells does it act through? (1)
(c) Why does an anti-histamine help? (1)
Hint
Not a fungal infection.
Answer
(a) Allergy (hypersensitivity, allergic dermatitis). (b) IgE, acting through mast cells. (c) Mast cells release histamine, which causes redness, swelling and itching; the drug blocks histamine's action and relieves symptoms.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Which antibody crosses the placenta?Show answer
IgG.
2.Which antibody is found in colostrum?Show answer
IgA.
3.Name the two primary lymphoid organs.Show answer
Bone marrow and thymus.
4.Which immune response causes graft rejection?Show answer
Cell-mediated immunity (T lymphocytes).
5.What does MALT stand for?Show answer
Mucosa-associated lymphoid tissue.
6.Name an autoimmune disease.Show answer
Rheumatoid arthritis.

Immunity, Antibodies and Lymphoid Organs: frequently asked questions

What is the difference between innate and acquired immunity?

Innate immunity is non-specific, present at birth and has no memory; acquired immunity is pathogen-specific, develops after exposure and has memory.

Why is an antibody called H₂L₂?

Because it has two heavy and two light polypeptide chains.

What are the five types of antibodies?

IgG, IgA, IgM, IgD and IgE.

What is the difference between primary and secondary lymphoid organs?

In primary organs (bone marrow, thymus) lymphocytes form and mature; in secondary organs (spleen, lymph nodes, tonsils, Peyer's patches, appendix) they meet antigens and proliferate.

How does a vaccine work?

It introduces antigens that trigger a primary response and leave memory B and T cells, so a later infection meets a rapid, strong secondary response.