Immunity

BiologyHuman Health and DiseaseFor NEET aspirants

Immunity is the overall ability of the body to fight disease-causing organisms, and it is conferred by the immune system. This page explains innate and acquired immunity, the structure of an antibody, humoral and cell-mediated responses, active and passive immunity, vaccination, allergy, autoimmunity, lymphoid organs and MALT, and how HIV causes AIDS, following the NCERT Class 12 chapter Human Health and Disease. NEET often asks about the four innate barriers, antibody types, lymphoid organs and the HIV cycle in questions on immunity.

On this page1Immunity2Innate immunity3Acquired immunity4Active and passive5Vaccination6Allergy7Autoimmunity8Lymphoid organs9AIDS10Exam essentials11Quick revision12Solved examples13Practice

Key Points at a Glance

★ Must learn Immunity: the overall ability of the host to fight disease-causing organisms, conferred by the immune system.
Innate immunity: non-specific, present at birth; four barriers: physical, physiological, cellular and cytokine.
★ Must learn Acquired immunity: pathogen specific, with memory; weak primary response, intense secondary (anamnestic) response.
★ Must learn An antibody has 4 peptide chains, 2 light and 2 heavy, so it is written $H_{2} L_{2}$ ; types include IgA, IgM, IgE and IgG.
Humoral response: antibodies from B-lymphocytes; cell-mediated immunity (CMI): T-lymphocytes, which cause graft rejection.
Active immunity: the host makes antibodies (slow); passive immunity: ready-made antibodies (colostrum IgA, placenta, antitoxin).
Allergy: IgE antibodies; histamine and serotonin from mast cells.
Auto-immune disease: the body attacks self-cells, e.g., rheumatoid arthritis.
★ Must learn Primary lymphoid organs: bone marrow and thymus; secondary: spleen, lymph nodes, tonsils, Peyer's patches, appendix; MALT is about 50 per cent of lymphoid tissue.
★ Must learn AIDS: HIV, a retrovirus; reverse transcriptase makes viral DNA; macrophages act as an HIV factory; helper T-lymphocytes decline.
ELISA diagnoses AIDS; anti-retroviral drugs are only partially effective.
HIV spreads only through body fluids, not by touch or physical contact.

1. Immunity and its Types

Every day we are exposed to a large number of infectious agents, yet only a few of these exposures result in disease.
The reason: the body can defend itself from most of these foreign agents.

★ Very important Immunity: the overall ability of the host to fight the disease-causing organisms, conferred by the immune system.

Immunity is of two types: innate immunity and acquired immunity.

2. Innate Immunity

★ Exam imp Innate immunity: a non-specific type of defence that is present at the time of birth.
It works by providing different types of barriers to the entry of foreign agents into the body.
Innate immunity consists of four types of barriers.

Barrier	Examples	How it protects
Physical	Skin (the main barrier); mucus coating of the epithelium lining the respiratory, gastrointestinal and urogenital tracts	Skin prevents entry of micro-organisms; mucus traps microbes entering the body
Physiological	Acid in the stomach, saliva in the mouth, tears from the eyes	Prevent microbial growth
Cellular	Leukocytes (WBC): polymorpho-nuclear leukocytes (PMNL-neutrophils), monocytes and natural killer cells (a type of lymphocyte) in the blood; macrophages in tissues	Phagocytose and destroy microbes
Cytokine	Interferons: proteins secreted by virus-infected cells	Protect non-infected cells from further viral infection

Memory Trick

Please Protect Cells Constantly: Physical, Physiological, Cellular, Cytokine, in that order.

NEET Focus

Skin and mucus are physical; acid, saliva and tears are physiological. Natural killer cells are lymphocytes, yet they belong to the cellular barrier of innate immunity. Interferons are made by virus-infected cells but protect the non-infected cells.

Key idea

Innate immunity is non-specific and inborn, working through physical, physiological, cellular and cytokine barriers.

3. Acquired Immunity

3.1 Features

★ Exam imp Acquired immunity is pathogen specific and is characterised by memory.
On the first encounter with a pathogen, the body produces a primary response of low intensity.
A later encounter with the same pathogen produces a highly intensified secondary or anamnestic response.
This is because the body appears to have a memory of the first encounter.

3.2 B-lymphocytes and T-lymphocytes

The primary and secondary responses are carried out by two special types of lymphocytes in the blood: B-lymphocytes and T-lymphocytes.
★ Exam imp B-lymphocytes produce an army of proteins, called antibodies, into the blood to fight pathogens.
★ Exam imp T-cells themselves do not secrete antibodies, but they help B cells to produce them.

3.3 Structure of an antibody

Each antibody molecule has four peptide chains: two small light chains and two longer heavy chains.
★ Exam imp Hence an antibody is represented as $H_{2} L_{2}$ .
Each arm ends in an antigen binding site; S-S (disulphide) bridges link the chains.
The N ends of the chains lie at the tips of the arms; the C ends of the heavy chains lie at the base.
Different types of antibodies are produced in the body; IgA, IgM, IgE and IgG are some of them.

Test yourself: hide labels

Figure 1: Structure of an antibody molecule. Two heavy and two light chains form a Y, written H2L2, with an antigen binding site at the tip of each arm.

Memory Trick

GAME recalls the four antibody types named here: IgG, IgA, IgM, IgE. H2L2 means 2 Heavy + 2 Light = 4 chains.

Quick Recall: tap to check

How many peptide chains does an antibody have?

Four: two light chains and two heavy chains (

H_{2} L_{2}

Name the part at the tip of each arm of an antibody.

The antigen binding site.

Which chains run down into the stem of the Y?

The two heavy chains; their C ends lie at the base.

Which bonds join the chains of an antibody?

S-S (disulphide) bridges.

3.4 Humoral and cell-mediated immunity

Humoral immune response

Antibody-mediated.
Antibodies are found in the blood, hence 'humoral'.
Antibodies are produced by B-lymphocytes.

Cell-mediated immunity (CMI)

Mediated by T-lymphocytes.
Responsible for graft rejection.
The second type of acquired immune response.

3.5 Organ transplantation and graft rejection

When organs such as the heart, eye, liver or kidney fail, transplantation is often the only remedy for a normal life.
A search then begins for a suitable donor, and doctors check whether the donor's tissue and blood group match.
Grafts cannot be taken from just any source, such as an animal, another primate or any human being; they would be rejected sooner or later.
Tissue matching and blood group matching are essential before any graft or transplant.
Even after matching, the patient has to take immunosuppressants all his or her life.

★ Very important The body can differentiate 'self' from 'nonself'. The cell-mediated immune response is responsible for graft rejection.

Key idea

Acquired immunity is specific and remembers: B cells make antibodies (humoral), while T cells mediate CMI and graft rejection.

4. Active and Passive Immunity

Feature	Active immunity	Passive immunity
Source of antibodies	Produced in the host body after exposure to antigens	Ready-made antibodies given directly
Antigens	Living or dead microbes or other proteins	Not needed
Speed	Slow; takes time to give its full effective response	Immediate protection
Examples	Deliberate injection of microbes during immunisation; natural infection	Colostrum (IgA) in mother's milk; antibodies from mother to foetus through the placenta; antitoxin; antivenom

★ Exam imp Colostrum: the yellowish fluid secreted by the mother during the initial days of lactation.
It has abundant antibodies (IgA) to protect the infant; this is why mother's milk is essential for the new-born.
The foetus also receives some antibodies from the mother through the placenta during pregnancy.

Key idea

If the body makes the antibodies, immunity is active; if it receives them ready-made, immunity is passive.

5. Vaccination and Immunisation

The principle of immunisation or vaccination is based on the 'memory' of the immune system.
Vaccine: a preparation of antigenic proteins of a pathogen, or of an inactivated or weakened pathogen, introduced into the body.
Antibodies produced against these antigens neutralise the pathogenic agents during actual infection.
★ Exam imp Vaccines also generate memory B and T-cells. These recognise the pathogen quickly on later exposure and overwhelm the invaders with a massive production of antibodies.

5.1 Passive immunisation

Some deadly microbes need a quick immune response, as in tetanus.
Here, preformed antibodies or antitoxin (a preparation containing antibodies to the toxin) are injected directly.
In snakebites too, the injection contains preformed antibodies against the snake venom.
This type of immunisation is called passive immunisation.

5.2 Vaccines from recombinant DNA technology

Recombinant DNA technology allows antigenic polypeptides of a pathogen to be produced in bacteria or yeast.
Such vaccines allow large-scale production, and hence greater availability for immunisation.
★ Exam imp Example: hepatitis B vaccine produced from yeast.

Tips and Tricks

Made by you = active; received ready-made = passive. A vaccine makes the body produce antibodies and memory cells, so it gives active immunity. Antitoxin for tetanus, antivenom for snakebite, colostrum and placental antibodies give passive immunity.

Key idea

Vaccines rely on immune memory; antitoxins and antivenoms give instant, passive protection; recombinant vaccines such as hepatitis B come from yeast.

6. Allergies

Some people start sneezing or wheezing in a new place, and the symptoms disappear when they leave.
Such a reaction may be an allergy to pollen, mites, etc., which differ from place to place.

★ Very important Allergy: the exaggerated response of the immune system to certain antigens present in the environment. The substances causing it are called allergens.

★ Exam imp The antibodies produced against allergens are of the IgE type.
★ Exam imp Common allergens: mites in dust, pollens and animal dander.
Symptoms: sneezing, watery eyes, running nose and difficulty in breathing.
★ Exam imp Allergy is due to the release of chemicals such as histamine and serotonin from mast cells.
To find the cause, the patient is exposed to, or injected with, very small doses of possible allergens, and the reactions are studied.
Drugs such as anti-histamine, adrenalin and steroids quickly reduce the symptoms.
Modern-day life style has lowered immunity and increased sensitivity to allergens.
More and more children in metro cities of India suffer from allergies and asthma, possibly because of the protected environment provided early in life.

Memory Trick

E for Environment: allergy is a response to environmental antigens, and the antibody made is IgE. Mast cells release histamine and serotonin.

Key idea

Allergy is an exaggerated IgE response to environmental allergens, driven by histamine and serotonin from mast cells.

7. Auto Immunity

Memory-based acquired immunity evolved in higher vertebrates.
It is based on the ability to tell foreign organisms (e.g., pathogens) from self-cells; the basis of this ability is still not understood.
Corollary one: higher vertebrates can distinguish foreign molecules as well as foreign organisms. Most experimental immunology deals with this.
Corollary two: sometimes, due to genetic and other unknown reasons, the body attacks self-cells. This damages the body and is called an auto-immune disease.
★ Exam imp Rheumatoid arthritis, which affects many people in our society, is an auto-immune disease.

Key idea

Autoimmunity is a failure of self and nonself discrimination; rheumatoid arthritis is the standard example.

8. Immune System in the Body

The human immune system consists of lymphoid organs, tissues, cells and soluble molecules such as antibodies.
It is unique: it recognises foreign antigens, responds to them and remembers them.
It also plays an important role in allergic reactions, auto-immune diseases and organ transplantation.

Memory Trick

The immune system works by three Rs: Recognise, Respond, Remember.

8.1 Lymphoid organs

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

Feature	Primary lymphoid organs	Secondary lymphoid organs
Organs	Bone marrow, thymus	Spleen, lymph nodes, tonsils, Peyer's patches of the small intestine, appendix
Role	Immature lymphocytes differentiate into antigen-sensitive lymphocytes	Sites where lymphocytes interact with the antigen, then proliferate to become effector cells

After maturation, lymphocytes migrate from the primary to the secondary lymphoid organs.

Memory Trick

Lymphocytes are Born and Trained in B and T (Bone marrow and Thymus), then sent to work in the secondary organs, which STAPLe them to antigens: Spleen, Tonsils, Appendix, Peyer's patches, Lymph nodes.

8.2 The organs one by one

★ Exam imp Bone marrow: the main lymphoid organ, where all blood cells, including lymphocytes, are produced.
Thymus: a lobed organ located near the heart and beneath the breastbone.
★ Exam imp The thymus is quite large at birth, keeps reducing in size with age, and is very small by puberty.
Both bone marrow and thymus provide micro-environments for the development and maturation of T-lymphocytes.
Spleen: a large, bean-shaped organ containing mainly lymphocytes and phagocytes.
★ Exam imp The spleen acts as a filter of the blood by trapping blood-borne micro-organisms; it also has a large reservoir of erythrocytes.
Lymph nodes: small solid structures at different points along the lymphatic system.
Lymph nodes trap micro-organisms or other antigens that get into the lymph and tissue fluid.
Antigens trapped in the lymph nodes activate the lymphocytes present there and cause the immune response.

Extra Depth: B-lymphocytes complete their maturation in the bone marrow, and T-lymphocytes mature in the thymus.

Test yourself: hide labels

Figure 2: Lymph nodes, thymus and lymphatic vessels in the human body. Lymph nodes sit at points along the lymphatic vessels and trap antigens from the lymph.

8.3 Mucosa-associated lymphoid tissue

★ Very important Mucosa-associated lymphoid tissue (MALT): lymphoid tissue located within the lining of the major tracts (respiratory, digestive and urogenital). It forms about 50 per cent of the lymphoid tissue in the human body.

Quick Recall: tap to check

Name the two primary lymphoid organs.

Bone marrow and thymus.

Which organ filters blood and also stores erythrocytes?

The spleen.

What happens to the size of the thymus with age?

It is large at birth, keeps shrinking with age, and is very small by puberty.

What share of the body's lymphoid tissue is MALT?

About 50 per cent.

Key idea

Bone marrow produces lymphocytes, and bone marrow and thymus mature them; spleen, lymph nodes, tonsils, Peyer's patches, appendix and MALT are where they meet antigens.

9. AIDS

9.1 What AIDS is

★ Exam imp AIDS: Acquired Immuno Deficiency Syndrome.
It is a deficiency of the immune system acquired during the lifetime of an individual, so it is not a congenital disease.
'Syndrome' means a group of symptoms.
AIDS was first reported in 1981. In about twenty-five years it spread all over the world, killing more than 25 million persons.
Cause: the Human Immuno deficiency Virus (HIV).
★ Exam imp HIV belongs to a group of viruses called retroviruses, which have an envelope enclosing the RNA genome.

9.2 Transmission and high-risk groups

(a) Sexual contact with an infected person.
(b) Transfusion of contaminated blood and blood products.
(c) Sharing infected needles, as by intravenous drug abusers.
(d) From an infected mother to her child through the placenta.
High-risk groups: people with multiple sexual partners, drug addicts who take drugs intravenously, people who need repeated blood transfusions, and children born to an HIV-infected mother.
★ Exam imp HIV/AIDS is not spread by mere touch or physical contact; it spreads only through body fluids.
So, for their physical and psychological well-being, HIV/AIDS-infected persons should not be isolated from family and society.
★ Exam imp There is always a time-lag between infection and the appearance of AIDS symptoms: a few months to many years, usually 5-10 years.

Memory Trick

Four routes of HIV: Some Bad Needle Practices: Sexual contact, Blood transfusion, Needles shared, Placenta.

9.3 How HIV destroys immunity

After getting into the body, the virus enters macrophages.
In the macrophage, the RNA genome of the virus replicates to form viral DNA with the help of the enzyme reverse transcriptase.
The viral DNA gets incorporated into the host cell's DNA and directs the infected cells to produce virus particles.
The macrophages continue to produce virus and in this way act like an HIV factory.
Simultaneously, HIV enters helper T-lymphocytes ( $T_{H}$ ), replicates and produces progeny viruses.
Progeny viruses released into the blood attack other helper T-lymphocytes. This is repeated, causing a progressive decrease in their number.
During this period, the person suffers from bouts of fever, diarrhoea and weight loss.
With fewer helper T-lymphocytes, the person suffers infections that could otherwise be overcome: bacteria (especially Mycobacterium), viruses, fungi and even parasites such as Toxoplasma.
The patient becomes so immuno-deficient that he or she cannot protect himself or herself against these infections.

Test yourself: hide labels

Figure 3: Replication of a retrovirus. Reverse transcriptase copies the viral RNA into DNA, which joins the host genome and directs the cell to make new viruses.

In the figure: the retrovirus has a viral RNA core inside a viral protein coat.
Inside the animal cell, viral RNA is introduced, viral DNA is made by reverse transcriptase and incorporates into the host genome in the nucleus.
The infected cell then produces new viral RNA and new viruses, which can infect other cells.
★ Exam imp The infected cell can survive while viruses are being replicated and released.

NEET Focus

RNA to DNA is the reverse of the usual flow, which is why HIV is a retrovirus and its enzyme is reverse transcriptase. The macrophage survives and keeps making virus (an HIV factory), while the helper T-lymphocytes are the cells whose number falls. Opportunistic infections, not HIV alone, overwhelm the patient.

9.4 Diagnosis and treatment

★ Exam imp A widely used diagnostic test for AIDS is ELISA (enzyme linked immuno-sorbent assay).
Treatment with anti-retroviral drugs is only partially effective.
These drugs can only prolong the life of the patient; they cannot prevent death.

9.5 Prevention of AIDS

AIDS has no cure, so prevention is the best option.
HIV infection more often spreads through conscious behaviour patterns, unlike pneumonia or typhoid, which can be caught inadvertently.
Infection in blood transfusion patients and new-borns (from the mother) may happen due to poor monitoring.
The only excuse may be ignorance, and it has been rightly said: "don't die of ignorance".
In India, the National AIDS Control Organisation (NACO) and non-governmental organisations (NGOs) educate people about AIDS.
WHO has started a number of programmes to prevent the spread of HIV infection.
Steps taken: making blood from blood banks safe from HIV; using only disposable needles and syringes in public and private hospitals and clinics; free distribution of condoms; controlling drug abuse; advocating safe sex; promoting regular check-ups for HIV in susceptible populations.
Infection with HIV, or having AIDS, should not be hidden, since the infection may then spread to many more people.
HIV/AIDS-infected people need help and sympathy instead of being shunned by society.
The disease can be tackled only by society and the medical fraternity acting together; otherwise the chances of wider spread increase manifold.

Key idea

HIV turns RNA into DNA, uses macrophages as a factory and destroys helper T cells; prevention through safe blood, needles and behaviour is the best defence.

10. Exam Essentials

Pairs to Match

List I	List II
Skin, mucus coating	Physical barrier
Stomach acid, saliva, tears	Physiological barrier
Neutrophils, monocytes, NK cells, macrophages	Cellular barrier
Interferons	Cytokine barrier
B-lymphocytes	Antibodies; humoral immune response
T-lymphocytes	Cell-mediated immunity; graft rejection
IgA	Colostrum
IgE	Allergy
Mast cells	Histamine and serotonin
Rheumatoid arthritis	Auto-immune disease
Antitoxin	Tetanus (passive immunisation)
Hepatitis B vaccine	Produced from yeast
Spleen	Filters blood; reservoir of erythrocytes
MALT	About 50 per cent of lymphoid tissue
Reverse transcriptase	Viral RNA to viral DNA
ELISA	Diagnosis of AIDS

Exceptions

T-cells do not secrete antibodies; they help B cells to produce them.
Innate immunity is not pathogen specific; acquired immunity is.
Natural killer cells are lymphocytes, yet they are part of the cellular barrier of innate immunity.
AIDS is not a congenital disease, although HIV can pass from mother to child through the placenta.
HIV does not spread by touch or physical contact; only through body fluids.
The HIV-infected macrophage is not killed; it survives and keeps releasing virus.
Anti-retroviral drugs cannot cure AIDS; they only prolong life.
The thymus shrinks with age and is very small by puberty.

Numbers to Remember

Innate barriers: 4. Antibody chains: 4 (2 heavy, 2 light), $H_{2} L_{2}$ .
Antibody types named: 4 (IgA, IgM, IgE, IgG).
Primary lymphoid organs: 2; secondary lymphoid organs named: 5.
MALT: about 50 per cent of the lymphoid tissue in the human body.
AIDS first reported in 1981; more than 25 million deaths in about 25 years.
Routes of HIV transmission: 4.
Time-lag between HIV infection and AIDS symptoms: usually 5-10 years.

Examples to Remember

Type	Examples
Active immunity	Natural infection; vaccination (e.g., hepatitis B vaccine from yeast)
Passive immunity	Colostrum (IgA); placental antibodies; tetanus antitoxin; antivenom for snakebite
Allergens	Mites in dust, pollens, animal dander
Anti-allergy drugs	Anti-histamine, adrenalin, steroids
Opportunistic infections in AIDS	Bacteria (especially Mycobacterium), viruses, fungi, Toxoplasma

11. Quick Revision

Immunity: the ability of the host to fight pathogens, conferred by the immune system; innate or acquired.
Innate: non-specific, present at birth; skin, mucous membranes, antimicrobial substances in tears and saliva, phagocytic cells and interferons.
Interferons are secreted by virus-infected cells and protect non-infected cells.
Acquired: specific, with memory; weak primary response, intense secondary (anamnestic) response.
B-lymphocytes make antibodies; T-cells help them; antibody = $H_{2} L_{2}$ ; IgA, IgM, IgE, IgG.
Humoral = antibody-mediated; CMI = T-lymphocytes; CMI causes graft rejection; transplant patients need lifelong immunosuppressants.
Active immunity: host makes antibodies, slowly; passive: ready-made antibodies such as colostrum IgA.
Vaccines use immune memory; antitoxin for tetanus and antivenom for snakebite are passive immunisation.
Recombinant DNA vaccines, e.g., hepatitis B vaccine from yeast.
Allergy: IgE; histamine and serotonin from mast cells; anti-histamine, adrenalin, steroids.
Auto-immune disease: body attacks self-cells; rheumatoid arthritis.
Primary lymphoid organs: bone marrow, thymus; secondary: spleen, lymph nodes, tonsils, Peyer's patches, appendix; MALT about 50 per cent.
AIDS: HIV, a retrovirus; macrophages become HIV factories; helper T cells fall; opportunistic infections follow.
HIV spreads by sex, blood, shared needles and placenta; not by touch; time-lag usually 5-10 years.
ELISA diagnoses AIDS; anti-retroviral drugs only prolong life; NACO, NGOs and WHO work on prevention.

12. Solved Examples

Solved Example 1

Match List I with List II.
List I: A. Physical barrier, B. Physiological barrier, C. Cellular barrier, D. Cytokine barrier
List II: I. Interferons, II. Natural killer cells, III. Tears from the eyes, IV. Mucus coating of the respiratory tract
Choose the correct answer:
(A) A-IV, B-III, C-II, D-I
(B) A-III, B-IV, C-II, D-I
(C) A-IV, B-II, C-III, D-I
(D) A-I, B-III, C-II, D-IV

Solution:

Answer: (A). Mucus coating is a physical barrier (IV), tears are physiological (III), natural killer cells are cellular (II) and interferons are the cytokine barrier (I).

Solved Example 2

Read the statements.
A. An antibody has two heavy and two light chains.
B. T-lymphocytes secrete antibodies.
C. Colostrum contains abundant IgA.
D. Graft rejection is due to the humoral immune response.
E. The secondary immune response is more intense than the primary response.
Choose the correct answer:
(A) A, B and C only
(B) A, C and E only
(C) B, D and E only
(D) C, D and E only

Solution:

Answer: (B). B is wrong: T-cells do not secrete antibodies; they help B cells. D is wrong: graft rejection is due to cell-mediated immunity.

Solved Example 3

Arrange the events after HIV enters the body in the correct order.
A. Viral DNA is incorporated into the host cell's DNA
B. HIV enters macrophages
C. Reverse transcriptase forms viral DNA from viral RNA
D. Progeny viruses attack helper T-lymphocytes, whose number falls
Choose the correct answer:
(A) B, A, C, D
(B) C, B, A, D
(C) B, C, A, D
(D) B, C, D, A

Solution:

Answer: (C). HIV enters macrophages (B), its RNA is copied into DNA by reverse transcriptase (C), the DNA joins the host genome (A), and the progeny viruses go on to deplete helper T-lymphocytes (D).

Solved Example 4

Which of the following is NOT a secondary lymphoid organ?
(A) Spleen
(B) Peyer's patches
(C) Thymus
(D) Tonsils

Solution:

Answer: (C). The thymus, with the bone marrow, is a primary lymphoid organ, where immature lymphocytes become antigen-sensitive.

Solved Example 5

A person bitten by a snake is given an injection of preformed antibodies against the venom. The immunity gained is:
(A) Active immunity, because antibodies are involved
(B) Passive immunity, because ready-made antibodies are given
(C) Innate immunity, because it is non-specific
(D) Auto-immunity, because the body attacks self-cells

Solution:

Answer: (B). Ready-made antibodies injected directly give passive immunity; the person's own immune system does not make them.

13. Practice Questions

Practice Questions

Match List I with List II.
List I: A. IgE, B. IgA, C. Interferons, D. Reverse transcriptase
List II: I. Viral RNA to DNA, II. Allergy, III. Colostrum, IV. Protect non-infected cells from viruses
(A) A-II, B-III, C-IV, D-I
(B) A-III, B-II, C-IV, D-I
(C) A-II, B-IV, C-III, D-I
(D) A-II, B-III, C-I, D-IVAnswer: (A). IgE is the allergy antibody, IgA is in colostrum, interferons protect non-infected cells, and reverse transcriptase copies RNA into DNA.
Read the statements.
A. The thymus is large at birth and very small by puberty.
B. The spleen is a reservoir of erythrocytes.
C. MALT forms about 50 per cent of the lymphoid tissue.
D. Lymph nodes are primary lymphoid organs.
(A) A, B and C only
(B) A and D only
(C) B, C and D only
(D) A, B, C and DAnswer: (A). D is wrong: lymph nodes are secondary lymphoid organs.
Arrange the events of acquired immunity in the order they occur when a vaccinated person meets the pathogen.
A. Memory B and T-cells recognise the pathogen
B. The pathogen enters the body
C. Antibodies neutralise the pathogen
D. Massive production of antibodies
(A) B, A, D, C
(B) A, B, D, C
(C) B, D, A, C
(D) B, A, C, DAnswer: (A). The pathogen enters, memory cells recognise it, antibodies are produced massively, and they neutralise the invader.
Which of the following is NOT a route of HIV transmission?
(A) Sharing infected needles
(B) Shaking hands with an infected person
(C) Transfusion of contaminated blood
(D) Infected mother to child through the placentaAnswer: (B). HIV is not spread by mere touch or physical contact.
Statement I: Acquired immunity is pathogen specific and has memory.
Statement II: Innate immunity develops only after the first infection.
(A) Both statements are correct
(B) Both statements are incorrect
(C) Statement I is correct but Statement II is incorrect
(D) Statement I is incorrect but Statement II is correctAnswer: (C). Innate immunity is present at the time of birth.
Allergic symptoms are due to the release of:
(A) Interferons from virus-infected cells
(B) Histamine and serotonin from mast cells
(C) Antibodies from T-lymphocytes
(D) Adrenalin from macrophagesAnswer: (B). Mast cells release histamine and serotonin.
A widely used diagnostic test for AIDS is:
(A) Widal test
(B) Biopsy
(C) ELISA
(D) MRIAnswer: (C). ELISA, the enzyme linked immuno-sorbent assay.

Practice Questions: Short Answer

Discuss what 'a suitable gene' means in the context of DNA vaccines.Answer: A gene that codes for an antigenic protein (polypeptide) of the pathogen. When this protein is made, it triggers the immune response and memory against that pathogen, without causing the disease.
Name the primary and secondary lymphoid organs.Answer: Primary: bone marrow and thymus. Secondary: spleen, lymph nodes, tonsils, Peyer's patches of the small intestine and appendix.
Expand: (a) MALT (b) CMI (c) AIDS (d) NACO (e) HIV.Answer: (a) Mucosa-associated lymphoid tissue (b) Cell-mediated immunity (c) Acquired Immuno Deficiency Syndrome (d) National AIDS Control Organisation (e) Human Immuno deficiency Virus.
Differentiate innate and acquired immunity, and active and passive immunity, with examples.Answer: Innate: non-specific, present at birth (skin, stomach acid, phagocytes, interferons). Acquired: pathogen specific, with memory (antibodies after infection). Active: the host makes antibodies, slowly (vaccination, natural infection). Passive: ready-made antibodies are given (colostrum, placental antibodies, antitoxin).
Draw a well-labelled diagram of an antibody molecule.Answer: A Y-shaped molecule of two heavy chains and two light chains joined by S-S bridges. Label the antigen binding site at each arm tip, light chain, heavy chain, the N ends at the arm tips and the C ends at the base (see Figure 1).
When do people need repeated blood transfusions, which puts them at high risk of HIV infection?Answer: In conditions such as thalassemia, a genetic disorder in which defective haemoglobin is formed, and haemophilia, in which blood does not clot normally. Such patients often need repeated transfusions of blood or blood products.
What are the routes by which HIV is transmitted?Answer: Sexual contact with an infected person; transfusion of contaminated blood and blood products; sharing infected needles; from an infected mother to her child through the placenta.
By what mechanism does the AIDS virus cause deficiency of the immune system?Answer: HIV enters macrophages, where reverse transcriptase makes viral DNA that joins the host DNA, so the macrophage becomes an HIV factory. HIV also enters helper T-lymphocytes and multiplies; progeny viruses attack more helper T cells. Their number falls steadily, so the person cannot fight even ordinary infections.

Common Mistakes to Avoid

Watch out

Saying T-lymphocytes secrete antibodies. B-lymphocytes produce antibodies; T-cells only help them.
Calling graft rejection a humoral response. It is due to cell-mediated immunity.
Listing interferons as a cellular barrier. They form the cytokine barrier and are secreted by virus-infected cells.
Calling colostrum or antitoxin active immunity. Ready-made antibodies give passive immunity.
Placing lymph nodes or the spleen among primary lymphoid organs. Only bone marrow and thymus are primary.
Writing IgG as the allergy antibody. Allergy antibodies are of the IgE type.
Calling AIDS congenital. It is acquired during the lifetime of an individual.
Saying HIV kills the macrophage at once. The macrophage survives and acts as an HIV factory; helper T-lymphocytes decline.

Frequently Asked Questions

What are the four barriers of innate immunity?

Physical barriers are the skin and the mucus coating of the respiratory, gastrointestinal and urogenital tracts. Physiological barriers are stomach acid, saliva and tears. Cellular barriers are neutrophils, monocytes, natural killer cells and macrophages. The cytokine barrier is formed by interferons from virus-infected cells.

What is the difference between innate and acquired immunity?

Innate immunity is non-specific and present at birth, working through barriers to the entry of foreign agents. Acquired immunity is pathogen specific and has memory. Its first response is weak, while a later encounter with the same pathogen gives an intense secondary or anamnestic response through B and T lymphocytes.

Why is an antibody written as H2L2?

Each antibody molecule has four peptide chains: two longer heavy chains and two small light chains. Writing H2L2 shows two heavy and two light chains. The chains form a Y with an antigen binding site at each arm tip. IgA, IgM, IgE and IgG are some antibody types.

How are active and passive immunity different?

In active immunity the host produces antibodies after exposure to antigens, through infection or vaccination; it is slow but lasting. In passive immunity ready-made antibodies are given directly, as in colostrum (IgA), placental antibodies, tetanus antitoxin and antivenom. Passive protection is immediate.

What causes allergy?

Allergy is an exaggerated immune response to environmental antigens called allergens, such as dust mites, pollen and animal dander. IgE antibodies are produced, and mast cells release histamine and serotonin, causing sneezing, watery eyes, running nose and difficulty in breathing. Anti-histamine, adrenalin and steroids relieve the symptoms.

Which are the primary and secondary lymphoid organs?

Bone marrow and thymus are primary lymphoid organs, where immature lymphocytes become antigen-sensitive. The spleen, lymph nodes, tonsils, Peyer's patches of the small intestine and appendix are secondary lymphoid organs, where lymphocytes meet antigens and proliferate into effector cells. MALT forms about half the lymphoid tissue.

How does HIV cause AIDS?

HIV, a retrovirus, enters macrophages, where reverse transcriptase copies its RNA into DNA that joins the host DNA. The macrophage becomes an HIV factory. HIV also multiplies in helper T-lymphocytes and destroys them steadily. The person then cannot fight infections by bacteria such as Mycobacterium, viruses, fungi and Toxoplasma.

How can the spread of AIDS be prevented?

AIDS has no cure, so prevention is best. Blood from blood banks is made safe, only disposable needles and syringes are used, condoms are distributed free, drug abuse is controlled, safe sex is advocated and susceptible groups are checked regularly. NACO, NGOs and WHO educate people, and infected people need support, not isolation.

Previous year questions on Immunity

9 questions from past papers, each with a step-by-step solution.

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