How Your Body Remembers a Germ It Met Long Ago
Understand innate and acquired immunity with humoral and cell-mediated responses, the structure and classes of antibodies, the cause, spread, diagnosis and prevention of AIDS, and the effects of alcohol, smoking and drugs of abuse with ways to prevent misuse.
How does the body defend itself against germs?
Every day you meet bacteria, viruses and fungi, yet most never make you ill. Barriers keep many out, defence cells destroy others, and the immune system remembers past invaders so it can respond faster the next time.
This lesson covers innate and acquired immunity, antibodies, AIDS, and the effects of alcohol, smoking and drugs.
This lesson covers innate and acquired immunity, antibodies, AIDS, and the effects of alcohol, smoking and drugs.
How is innate immunity different from acquired immunity, and what are humoral and cell-mediated responses?
Innate immunity is non-specific defence present from birth, while acquired immunity is specific, develops after exposure to a pathogen and has memory; it works through antibodies made by B cells (the humoral response) and through T cells that act directly (the cell-mediated response).
Innate immunity — four barriers:
- Physical — skin and the mucus lining of the respiratory, digestive and urinogenital tracts
- Physiological — acid in the stomach, saliva and tears
- Cellular — neutrophils, monocytes, natural killer cells and macrophages that destroy microbes
- Cytokine — virus-infected cells release interferons that protect uninfected cells
Acquired immunity:
- Specific — targets a particular pathogen
- Memory — a first encounter gives a slow, weak primary response; a later encounter gives a fast, strong secondary response
- Humoral response — B lymphocytes make antibodies that circulate in blood and lymph
- Cell-mediated response — T lymphocytes attack infected or foreign cells and help B cells
Active and passive immunity:
- Active — the body makes its own antibodies after infection or vaccination; slow to develop but long-lasting
- Passive — ready-made antibodies are received, as from colostrum or an antivenom injection; quick but short-lived
An everyday example. Childhood vaccines given at government health centres train acquired immunity so the body responds quickly if the real pathogen arrives.
The substance. Vaccination works because of memory — it triggers a primary response safely, so a real infection meets a fast secondary response.
Innate immunity — four barriers:
- Physical — skin and the mucus lining of the respiratory, digestive and urinogenital tracts
- Physiological — acid in the stomach, saliva and tears
- Cellular — neutrophils, monocytes, natural killer cells and macrophages that destroy microbes
- Cytokine — virus-infected cells release interferons that protect uninfected cells
Acquired immunity:
- Specific — targets a particular pathogen
- Memory — a first encounter gives a slow, weak primary response; a later encounter gives a fast, strong secondary response
- Humoral response — B lymphocytes make antibodies that circulate in blood and lymph
- Cell-mediated response — T lymphocytes attack infected or foreign cells and help B cells
Active and passive immunity:
- Active — the body makes its own antibodies after infection or vaccination; slow to develop but long-lasting
- Passive — ready-made antibodies are received, as from colostrum or an antivenom injection; quick but short-lived
An everyday example. Childhood vaccines given at government health centres train acquired immunity so the body responds quickly if the real pathogen arrives.
The substance. Vaccination works because of memory — it triggers a primary response safely, so a real infection meets a fast secondary response.
What is the structure of an antibody, and what are the classes of immunoglobulins?
**An antibody is a Y-shaped protein made of two heavy and two light polypeptide chains, written , with antigen-binding sites at the tips of its arms, and antibodies fall into five classes — IgA, IgD, IgE, IgG and IgM.
Structure:
- Four chains — two identical heavy chains and two identical light chains, joined by disulphide bonds
- Variable regions at the tips differ between antibodies and bind specific antigens
- Each antibody has two antigen-binding sites
Classes of immunoglobulins:
- IgG — the main antibody in blood; crosses the placenta and protects the foetus
- IgA — found in colostrum, saliva and tears; protects mucous surfaces
- IgM — the largest antibody, made early in the primary response
- IgE — involved in allergic reactions, releasing histamine from mast cells
- IgD — present on the surface of B cells, helping to activate them
An everyday example. Sneezing and watery eyes during the pollen season in Indian cities are allergic reactions driven by IgE antibodies.
The substance. A newborn is protected partly by antibodies from the mother** — IgG crosses the placenta, and IgA arrives in colostrum.
Structure:
- Four chains — two identical heavy chains and two identical light chains, joined by disulphide bonds
- Variable regions at the tips differ between antibodies and bind specific antigens
- Each antibody has two antigen-binding sites
Classes of immunoglobulins:
- IgG — the main antibody in blood; crosses the placenta and protects the foetus
- IgA — found in colostrum, saliva and tears; protects mucous surfaces
- IgM — the largest antibody, made early in the primary response
- IgE — involved in allergic reactions, releasing histamine from mast cells
- IgD — present on the surface of B cells, helping to activate them
An everyday example. Sneezing and watery eyes during the pollen season in Indian cities are allergic reactions driven by IgE antibodies.
The substance. A newborn is protected partly by antibodies from the mother** — IgG crosses the placenta, and IgA arrives in colostrum.
What causes AIDS, how does it spread, and how is it diagnosed and prevented?
AIDS, the acquired immunodeficiency syndrome, is caused by HIV, a retrovirus that destroys helper T cells; it spreads through infected blood, unprotected sexual contact, shared needles and from an infected mother to the child, and it is diagnosed by ELISA and prevented by safe practices.
How HIV acts:
- HIV enters macrophages, where reverse transcriptase copies its RNA into viral DNA
- The viral DNA joins the host cell's DNA, and the macrophage keeps making viruses — an HIV factory
- HIV then enters helper T lymphocytes, multiplies and destroys them
- As helper T cells fall, the person becomes open to infections by bacteria such as Mycobacterium, viruses, fungi and parasites such as Toxoplasma
Transmission:
- Unprotected sexual contact with an infected person
- Transfusion of contaminated blood
- Sharing infected needles
- From an infected mother to the child through the placenta
Diagnosis and treatment. HIV infection is diagnosed by the ELISA test; antiretroviral drugs prolong life but cannot cure AIDS.
Prevention:
- Screening donated blood for HIV
- Using disposable needles and syringes
- Using condoms and avoiding unsafe sexual practices
An everyday example. Red ribbon awareness campaigns in Indian schools and colleges spread correct information about how HIV does and does not spread.
The substance. HIV does not spread by touching, hugging or sharing food — it needs body fluids such as blood to enter another person's bloodstream.
How HIV acts:
- HIV enters macrophages, where reverse transcriptase copies its RNA into viral DNA
- The viral DNA joins the host cell's DNA, and the macrophage keeps making viruses — an HIV factory
- HIV then enters helper T lymphocytes, multiplies and destroys them
- As helper T cells fall, the person becomes open to infections by bacteria such as Mycobacterium, viruses, fungi and parasites such as Toxoplasma
Transmission:
- Unprotected sexual contact with an infected person
- Transfusion of contaminated blood
- Sharing infected needles
- From an infected mother to the child through the placenta
Diagnosis and treatment. HIV infection is diagnosed by the ELISA test; antiretroviral drugs prolong life but cannot cure AIDS.
Prevention:
- Screening donated blood for HIV
- Using disposable needles and syringes
- Using condoms and avoiding unsafe sexual practices
An everyday example. Red ribbon awareness campaigns in Indian schools and colleges spread correct information about how HIV does and does not spread.
The substance. HIV does not spread by touching, hugging or sharing food — it needs body fluids such as blood to enter another person's bloodstream.
What are the effects of alcohol, smoking and drugs of abuse, and how can misuse be prevented?
Alcohol, tobacco and drugs such as opioids, cannabinoids and cocaine alter the nervous system and damage organs, can lead to addiction and dependence, and are best prevented through awareness, support from family and teachers, and early professional help.
Drugs of abuse:
- Opioids — bind receptors in the nervous system and gut; heroin, made from morphine, is a depressant that slows body functions
- Cannabinoids — obtained from Cannabis sativa as marijuana, hashish, charas and ganja; affect the heart and circulation
- Coca alkaloids — cocaine interferes with the transport of dopamine, causing euphoria and, in excess, hallucinations
Effects of alcohol. Long-term use damages the liver, causing cirrhosis, harms the nervous system, and leads to reckless and violent behaviour.
Effects of smoking:
- Nicotine stimulates the adrenal glands, raising blood pressure and heart rate
- Raises the risk of cancers of the lung, throat and mouth, bronchitis, emphysema and heart disease
Addiction and dependence. Repeated use creates a psychological attachment, and the body comes to depend on the drug, so stopping causes withdrawal symptoms.
Prevention:
- Resisting peer pressure and unrealistic expectations
- Seeking help from parents, teachers and trusted friends
- Medical help from de-addiction centres
An everyday example. De-addiction and counselling centres in many Indian districts help people recover from alcohol and drug dependence with medical and emotional support.
The substance. Adolescence is the most vulnerable stage — curiosity and peer pressure often lead to experimenting, which is why prevention focuses on young people.
Drugs of abuse:
- Opioids — bind receptors in the nervous system and gut; heroin, made from morphine, is a depressant that slows body functions
- Cannabinoids — obtained from Cannabis sativa as marijuana, hashish, charas and ganja; affect the heart and circulation
- Coca alkaloids — cocaine interferes with the transport of dopamine, causing euphoria and, in excess, hallucinations
Effects of alcohol. Long-term use damages the liver, causing cirrhosis, harms the nervous system, and leads to reckless and violent behaviour.
Effects of smoking:
- Nicotine stimulates the adrenal glands, raising blood pressure and heart rate
- Raises the risk of cancers of the lung, throat and mouth, bronchitis, emphysema and heart disease
Addiction and dependence. Repeated use creates a psychological attachment, and the body comes to depend on the drug, so stopping causes withdrawal symptoms.
Prevention:
- Resisting peer pressure and unrealistic expectations
- Seeking help from parents, teachers and trusted friends
- Medical help from de-addiction centres
An everyday example. De-addiction and counselling centres in many Indian districts help people recover from alcohol and drug dependence with medical and emotional support.
The substance. Adolescence is the most vulnerable stage — curiosity and peer pressure often lead to experimenting, which is why prevention focuses on young people.
Exam tip
What earns full marks on immunity, AIDS and drug abuse?
Answer immunity questions in pairs — innate versus acquired, humoral versus cell-mediated, active versus passive — with one example for each side.
- Antibody: with two antigen-binding sites
- IgG crosses the placenta; IgA is in colostrum; IgE causes allergy
The trap. Calling an antivenom injection active immunity. Antivenom supplies ready-made antibodies, so it gives passive immunity.
- Antibody: with two antigen-binding sites
- IgG crosses the placenta; IgA is in colostrum; IgE causes allergy
The trap. Calling an antivenom injection active immunity. Antivenom supplies ready-made antibodies, so it gives passive immunity.
Did you know
Why does chickenpox rarely strike the same person twice?
After chickenpox, the immune system keeps memory B cells and T cells that recognise the virus. If the same virus appears again, these cells respond so fast that the infection is usually stopped before any symptoms show.
That is the secondary immune response at work, and it is the same principle that vaccines rely on.
That is the secondary immune response at work, and it is the same principle that vaccines rely on.
Exam relevance
How does NEET test immunity, antibodies and AIDS?
Human Health and Disease is a recurring NEET chapter, and its immunity section is rich in comparisons.
What gets asked. The four barriers of innate immunity, primary and secondary responses, active versus passive immunity, antibody structure and immunoglobulin classes, how HIV multiplies in macrophages and helper T cells, and the sources and effects of opioids, cannabinoids and cocaine.
Question types. Mostly statement-based and match-the-column questions, often pairing a drug with its source or an immune cell with its role.
Why it matters later. ELISA, vaccines and antibody-based diagnosis return in Biotechnology and its Applications.
The trap that costs marks. Saying HIV attacks B cells first — HIV multiplies in macrophages and destroys helper T cells, weakening both humoral and cell-mediated responses.
What gets asked. The four barriers of innate immunity, primary and secondary responses, active versus passive immunity, antibody structure and immunoglobulin classes, how HIV multiplies in macrophages and helper T cells, and the sources and effects of opioids, cannabinoids and cocaine.
Question types. Mostly statement-based and match-the-column questions, often pairing a drug with its source or an immune cell with its role.
Why it matters later. ELISA, vaccines and antibody-based diagnosis return in Biotechnology and its Applications.
The trap that costs marks. Saying HIV attacks B cells first — HIV multiplies in macrophages and destroys helper T cells, weakening both humoral and cell-mediated responses.
Key takeaways
What must you be able to do from this lesson?
- Immunity: innate barriers, acquired immunity with memory, humoral and cell-mediated responses, and active versus passive immunity
- Antibodies: the structure and the classes IgA, IgD, IgE, IgG and IgM
- AIDS: HIV as a retrovirus, spread through body fluids, diagnosed by ELISA and prevented by safe practices
- Drugs and alcohol: effects on the body, addiction, and ways to prevent misuse
Why does a childhood vaccine protect you for a long time, while an antivenom injection protects for only a short time?
- Antibodies: the structure and the classes IgA, IgD, IgE, IgG and IgM
- AIDS: HIV as a retrovirus, spread through body fluids, diagnosed by ELISA and prevented by safe practices
- Drugs and alcohol: effects on the body, addiction, and ways to prevent misuse
Why does a childhood vaccine protect you for a long time, while an antivenom injection protects for only a short time?