BiologyCBSE Class 12 · Human Health and Disease

Immunity: Innate, Acquired, Active and Passive

Updated 15 October 2026 · Fact-checked

Immunity is the body's ability to fight pathogens. Innate immunity is non-specific and present from birth: physical, physiological, cellular and cytokine barriers. Acquired immunity is pathogen-specific and has memory, using B cells (antibodies) and T cells. Active immunity comes from the body's own antibodies after infection or vaccination; passive immunity from ready-made antibodies.

Notice that only acquired immunity has memory, and active means your own antibodies while passive means ready-made antibodies.

Illustration drawn for QuestionBuddy.

Understand Immunity: Innate, Acquired, Active and Passive

Two kinds of immunity. Immunity is the overall ability of the host to fight disease-causing organisms. It is innate or acquired.

Innate immunity is non-specific and present at birth. It works through four kinds of barriers:
1. Physical: the skin; mucus lining the respiratory, gastrointestinal and urogenital tracts traps microbes.
2. Physiological: acid in the stomach, saliva in the mouth and tears prevent microbial growth.
3. Cellular: leucocytes such as neutrophils (PMNL), monocytes and natural killer cells (a kind of lymphocyte) in blood, and macrophages in tissues, phagocytose and destroy microbes.
4. Cytokine: virus-infected cells secrete interferons, which protect non-infected cells from further viral infection.

Acquired immunity is pathogen-specific and has memory.

The first encounter gives a primary response of low intensity; a later encounter with the same pathogen gives a highly intensified secondary (anamnestic) response.
Two kinds of lymphocytes carry this out. B-lymphocytes produce antibodies into the blood: the humoral immune response.
T-lymphocytes do not secrete antibodies but help B cells produce them, and they mediate the cell-mediated immune response (CMI).
Antibody structure: four polypeptide chains, two light and two heavy, so an antibody is written ** $H_{2} L_{2}$ . Types: IgA, IgM, IgE, IgG**.
CMI and transplants: the body tells 'self' from 'non-self'. CMI is responsible for graft rejection, so tissue and blood group matching are essential before transplants of organs such as heart, eye, liver or kidney, and the patient must take immunosuppressants for life.

Active vs passive immunity.

Active: the host is exposed to antigens (living or dead microbes or other proteins) and makes its own antibodies. It is slow to develop fully but long-lasting, with memory. Induced by natural infection or by deliberate immunisation.
Passive: ready-made antibodies are given. It is immediate but short-lived, with no memory. Examples: colostrum (yellowish first milk) is rich in IgA antibodies that protect the infant; the fetus receives some antibodies through the placenta; for a quick response (e.g. tetanus, snakebite) preformed antibodies or antitoxin are injected; antivenom is given to snakebite victims.

Vaccination and immunisation rely on memory. A vaccine (antigenic proteins of a pathogen, or an inactivated or weakened pathogen) is introduced. Antibodies made in response neutralise the pathogen during a real infection, and memory B and T cells recognise it quickly next time and overwhelm it with a massive antibody response. Recombinant DNA technology produces antigenic polypeptides in bacteria or yeast, allowing large-scale production: e.g. the hepatitis B vaccine from yeast.

When the system misfires.

Allergy: an exaggerated response to certain environmental antigens (allergens) such as dust mites, pollen and animal dander. IgE antibodies are produced; mast cells release histamine and serotonin, causing sneezing, watery eyes, running nose and breathing difficulty. Treated with anti-histamines, adrenalin and steroids. Allergens are identified by exposing the patient to very small doses of possible allergens.
Autoimmunity: due to genetic and other unknown reasons, the body attacks self-cells, e.g. rheumatoid arthritis.

Lymphoid organs.

Primary: bone marrow (all blood cells, including lymphocytes, are produced here) and thymus (a lobed organ near the heart beneath the breastbone, large at birth and shrinking to very small by puberty). T-lymphocytes develop and mature in the special micro-environments that both bone marrow and thymus provide; immature lymphocytes become antigen-sensitive here.
Secondary: spleen (bean-shaped; lymphocytes and phagocytes; filters blood by trapping blood-borne microbes; reservoir of RBCs), lymph nodes (trap microbes and antigens in lymph and tissue fluid and activate lymphocytes), tonsils, Peyer's patches of the small intestine and the appendix. Here lymphocytes meet antigens and proliferate into effector cells.
MALT (mucosa-associated lymphoid tissue) in the linings of the respiratory, digestive and urogenital tracts makes up about 50 per cent of the lymphoid tissue in the body.

Flow chart (acquired response): antigen enters → B and T cells recognise it → primary response (slow, low) + memory cells → same antigen again → secondary response (fast, strong) → pathogen cleared.

Draw and label: structure of an antibody. Checklist: a Y-shaped molecule of four chains: two long inner chains forming the stem and arms (heavy chains), two short outer chains along the arms (light chains), short bars linking chains, binding pockets at both arm tips. Labels: heavy chain, light chain, antigen-binding site, S-S (disulphide) bonds; write $H_{2} L_{2}$ beside it.

Immunity: Innate, Acquired, Active and Passive: key facts

Antibody: $H_{2} L_{2} : 2 heavy + 2 light chains; types I g A, I g M, I g E, I g G$; Two antigen-binding sites per molecule.
Innate barriers: $physical ∣ physiological ∣ cellular ∣ cytokine$; Skin/mucus; acid, saliva, tears; neutrophils, monocytes, NK cells, macrophages; interferons.
Active vs passive: $active: own antibodies, slow, lasting; passive: ready-made, fast, short$; Vaccination is active; colostrum, antitoxin and antivenom give passive immunity.
Humoral vs cell-mediated: $B cells \to antibodies (humoral); T cells \to CMI, graft rejection$; T helper cells help B cells make antibodies.
Lymphoid organs: $primary: bone marrow, thymus; secondary: spleen, lymph nodes, tonsils, Peyer’s patches, appendix$; MALT is about 50% of lymphoid tissue.

How to answer Immunity: Innate, Acquired, Active and Passive questions

Immunity questions test classification and reasoning. Decide which branch of immunity a case belongs to, then explain the mechanism.

1Ask: specific or non-specific? Present at birth or acquired? Non-specific barriers = innate; specific with memory = acquired.
2For innate barriers, name the type and give the example (skin = physical; tears = physiological; macrophages = cellular; interferons = cytokine).
3For active vs passive, ask who made the antibodies: the person's own body (active) or someone else (passive).
4For speed questions (snakebite, tetanus), choose passive immunity because ready-made antibodies act at once; for long-term protection choose vaccination (active).
5For graphs of antibody level, mark the primary response (low, slow) and secondary response (high, fast) and explain by memory cells.
6For transplant or allergy cases, name the response: CMI (graft rejection) or IgE with histamine from mast cells (allergy).

Immunity: Innate, Acquired, Active and Passive 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.

Choosing active or passive immunity

Suppose a farmer is bitten by a venomous snake while working in a field, and the same week his young son is due for his routine vaccination against hepatitis B.

Which type of immunity should each receive, in what form, and why would the opposite choice fail?

Try it first, then show the solutionSolution

Farmer: passive immunity, by antivenom (preformed antibodies against the venom). Venom acts fast, and his body would take too long to make its own antibodies.
Son: active immunity, by the hepatitis B vaccine (made in yeast by recombinant DNA technology). His B and T cells make antibodies and memory cells for long-term protection.
Opposite choices: a vaccine would be far too slow to save the farmer; antibodies injected into the son would protect only briefly and leave no memory.

Answer: Farmer: antivenom (passive, immediate). Son: vaccine (active, long-lasting with memory).

Memory hook: Emergency: borrow antibodies. Prevention: build your own.

Tricks to remember Immunity: Innate, Acquired, Active and Passive

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

Innate barriers: P-P-C-C
Physical, Physiological, Cellular, Cytokine.
Use it when: Listing the four barriers.
B for Blood antibodies, T for Tackling cells
B cells make antibodies (humoral, in blood); T cells handle cell-mediated immunity and help B cells.
Use it when: Humoral vs CMI.
IgE = Itchy Eyes
IgE is the allergy antibody; histamine makes eyes water.
Use it when: Allergy questions.

Immunity: Innate, Acquired, Active and Passive: solved examples

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

Example 1: Four innate barriers · 2 marks · Very short answerBoard level

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

Try it first, then show the solutionSolution

Physical: skin or mucus lining of tracts; physiological: stomach acid, saliva or tears (1 mark).
Cellular: neutrophils, monocytes, NK cells or macrophages; cytokine: interferons from virus-infected cells (1 mark).

Answer: Physical (skin), physiological (tears), cellular (macrophages), cytokine (interferons).

Example 2: Antibody structure · 3 marks · Short answerBoard level

Draw a labelled diagram of an antibody molecule. Why is it represented as $H_{2} L_{2}$ ? Name the type of antibody produced in allergy.

Try it first, then show the solutionSolution

Diagram: Y-shaped molecule with two heavy chains, two light chains, antigen-binding sites at the tips and disulphide bonds (1½ marks).
It has four polypeptide chains: two heavy (H) and two light (L), hence $H_{2} L_{2}$ (1 mark).
IgE (½ mark).

Answer: Y-shaped, 2 heavy + 2 light chains; $H_{2} L_{2}$ ; IgE in allergy.

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

Suppose a graph shows antibody levels after a person meets a pathogen on day 0 and again on day 40. The first peak is small and appears slowly; the second is much higher and appears quickly. Explain the difference and relate it to vaccination.

Try it first, then show the solutionSolution

First exposure: primary response of low intensity, because only a few specific B and T cells exist and take time to multiply (1 mark).
During it, memory B and T cells are formed; on the second exposure they recognise the pathogen at once, giving the highly intensified secondary (anamnestic) response (1 mark).
A vaccine acts like the first exposure: it creates memory cells, so a real infection later meets a fast, strong secondary response (1 mark).

Answer: Primary: slow and small; secondary: fast and large due to memory cells; vaccination exploits this memory.

Example 4: Colostrum and placenta · 2 marks · Very short answerHarder board level

Why is breastfeeding during the first days after birth important for the newborn? Which other route gives the baby antibodies before birth? What type of immunity is this?

Try it first, then show the solutionSolution

Colostrum, the yellowish milk of the first days of lactation, has abundant IgA antibodies that protect the infant (1 mark).
The fetus receives some antibodies from the mother through the placenta. Both are passive immunity (ready-made antibodies) (1 mark).

Answer: Colostrum (IgA) and placental antibodies give passive immunity.

Example 5: Transplant rejection · 3 marks · Short answerHOTS / competency

Suppose a patient receives a kidney from a donor whose tissue type is not matched. Explain which arm of immunity causes rejection, and why even matched recipients take medicines for life.

Try it first, then show the solutionSolution

The immune system distinguishes 'self' from 'non-self'; the donor kidney is recognised as non-self (1 mark).
T-lymphocytes mediate the cell-mediated immune response that attacks the graft: graft rejection (1 mark).
Even with tissue and blood group matching, the match is not perfect, so patients take immunosuppressants for life to suppress the response (1 mark).

Answer: CMI by T cells rejects non-self tissue; immunosuppressants prevent rejection.

Example 6: Lymphoid organs · 3 marks · Short answerHarder board level

Differentiate between primary and secondary lymphoid organs with two examples each. Why does the spleen act as a blood filter?

Try it first, then show the solutionSolution

Primary lymphoid organs (bone marrow, thymus): immature lymphocytes develop and mature into antigen-sensitive lymphocytes (1 mark).
Secondary lymphoid organs (spleen, lymph nodes, tonsils, Peyer's patches, appendix): mature lymphocytes interact with antigens and proliferate into effector cells (1 mark).
The spleen contains lymphocytes and phagocytes that trap and remove blood-borne microorganisms; it also stores RBCs (1 mark).

Answer: Primary: maturation (bone marrow, thymus); secondary: antigen interaction (spleen, lymph nodes); spleen phagocytes trap microbes from blood.

Where students lose marks in Immunity: Innate, Acquired, Active and Passive

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

Mistake: Calling vaccination passive immunity
Students think anything injected is passive.
Wrong working
Vaccination gives passive immunity.
Right working
Vaccination gives active immunity: the body makes its own antibodies and memory cells.
Do this: Ask who makes the antibodies.
Usually costs: 1 mark.
Mistake: Saying T cells produce antibodies
B and T roles are swapped.
Wrong working
T-lymphocytes secrete antibodies.
Right working
B cells secrete antibodies; T cells help B cells and mediate CMI.
Do this: B for blood antibodies.
Usually costs: 1 mark.
Mistake: Wrong antibody notation
Students count chains wrongly.
Wrong working
Antibody = $H_{1} L_{1}$ or $H_{4} L_{2}$ .
Right working
$H_{2} L_{2}$ : two heavy and two light chains.
Do this: Draw the Y and count: two long, two short.
Usually costs: ½-1 mark.
Mistake: Listing the spleen as a primary lymphoid organ
Organs are memorised without function.
Wrong working
Primary lymphoid organs: thymus and spleen.
Right working
Primary: bone marrow and thymus. Spleen is secondary.
Do this: Primary = where lymphocytes are made or mature.
Usually costs: 1 mark.

Exam tips

Use the terms humoral and cell-mediated immune response in acquired immunity answers.
For passive immunity give at least two examples: colostrum (IgA), placental antibodies, antitoxin, antivenom.
Mention memory cells whenever vaccination is asked.
Name MALT and its 50 per cent share when asked about lymphoid tissue.
Immunology is tested only at the level of basic concepts; avoid going beyond NCERT detail.

Practice questions on Immunity: Innate, Acquired, Active and Passive

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
Interferons are an example of which barrier of innate immunity?
(a) Physical (b) Physiological (c) Cellular (d) Cytokine
Hint
Secreted proteins.
Answer
(d) Cytokine.
Q2 · 1 mark · Assertion-reasonBoard level
Assertion (A): Antivenom injection gives immediate protection to a snakebite victim.
Reason (R): Antivenom contains preformed antibodies, providing passive immunity.
(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
Who made the antibodies?
Answer
(a).
Q3 · 3 marks · Short answerHarder board level
Explain the symptoms of an allergy in terms of the cells and chemicals involved, and name three drugs that reduce them.
Hint
IgE, mast cells.
Answer
Allergens (dust mites, pollen, animal dander) trigger IgE antibody production; mast cells release chemicals such as histamine and serotonin, which cause sneezing, watery eyes, running nose and difficulty in breathing. Drugs: anti-histamines, adrenalin and steroids.
Q4 · 2 marks · Very short answerHOTS / competency
Suppose a child's thymus is examined at birth and again at 15 years. What change would be seen, and why does the immune system still work well in the teenager?
Hint
Thymus at puberty; memory and mature T cells.
Answer
The thymus is large at birth but shrinks to a very small size by puberty. The immune system still works because T-lymphocytes have already matured and populated the secondary lymphoid organs, and bone marrow continues to produce lymphocytes; memory cells built over childhood also persist.
Q5 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose a doctor tells a group of students: 'Your skin, your tears and your stomach acid protect you every day; but measles can affect you only once in a lifetime, and the vaccine you took as a baby is why you never got hepatitis B.'
(a) Which statements refer to innate immunity? (1)
(b) Why does measles not recur? (1)
(c) Is the hepatitis B vaccine an example of active or passive immunity? How was this vaccine produced on a large scale? (2)
Hint
Memory; recombinant DNA in yeast.
Answer
(a) Skin (physical barrier), tears and stomach acid (physiological barriers). (b) The first infection leaves memory B and T cells, so a second exposure triggers a fast, strong secondary response that stops the disease. (c) Active immunity (the body makes its own antibodies). It is produced using recombinant DNA technology: the antigenic polypeptide of the virus is made in yeast, allowing large-scale production.
Q6 · 2 marks · Very short answerHarder board level
What is autoimmunity? Give one example and explain what has gone wrong.
Hint
Self vs non-self.
Answer
Autoimmunity is when, due to genetic and other unknown reasons, the immune system attacks the body's own cells. Example: rheumatoid arthritis. The normal ability to distinguish self from non-self fails.

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 is found in colostrum?Show answer
IgA.
2.Which cells mediate graft rejection?Show answer
T-lymphocytes (cell-mediated immunity).
3.What do virus-infected cells secrete to protect neighbouring cells?Show answer
Interferons.
4.Name the primary lymphoid organs.Show answer
Bone marrow and thymus.
5.What chemicals do mast cells release in allergy?Show answer
Histamine and serotonin.
6.Give an example of a vaccine made in yeast.Show answer
Hepatitis B vaccine.

Immunity: Innate, Acquired, Active and Passive: frequently asked questions

What is the difference between innate and acquired immunity?

Innate immunity is non-specific and present from birth (barriers); acquired immunity is specific to a pathogen, develops after exposure and has memory.

What is the difference between active and passive immunity?

In active immunity the body makes its own antibodies after infection or vaccination (slow but long-lasting); in passive immunity ready-made antibodies are given (fast but short-lived).

Why is an antibody called H2L2?

It has four polypeptide chains: two heavy (H) chains and two light (L) chains.

How does vaccination work?

A vaccine introduces antigens or a weakened or killed pathogen; the body makes antibodies and memory B and T cells, which respond quickly and strongly to a real infection.

What are secondary lymphoid organs?

Organs where mature lymphocytes meet antigens and multiply: spleen, lymph nodes, tonsils, Peyer's patches of the small intestine and the appendix.