Maarg Manthan · Topic 3.7

Immune System and Immunity

Science & Technology › Human Physiology – Organ Systems · Topic 3.7

The immune system is the network of cells, tissues and organs that protects the body from disease-causing microbes and from abnormal cells. Immunity is of two kinds. Innate immunity is present from birth and is non-specific. It includes barriers such as skin and mucus, phagocytic cells, inflammation, fever and natural killer cells. Adaptive immunity develops after exposure to a specific antigen, uses lymphocytes and has memory. B cells make antibodies and T cells destroy infected cells or help other cells. The key organs are the bone marrow, thymus, spleen and lymph nodes. Vaccines work by training adaptive immunity without causing disease, beginning with Edward Jenner's smallpox vaccine in 1796.

5 min read · UPSC Prelims · Science & Technology · Human Physiology – Organ Systems

At a Glance

Innate immunity Born with it; non-specific
Adaptive immunity Specific; has memory
Antibody makers B lymphocytes
Organs Bone marrow, thymus, spleen, lymph nodes
First vaccine Smallpox, Jenner 1796
Exam link UPSC Prelims, Science & Technology: Human Physiology – Organ Systems

Where Does It Fit?

This is the seventh topic of the macro on human physiology and organ systems. It describes how the body defends itself and prepares for the later macro on human health and diseases. Previous topic: Digestive System and Human Dietary Patterns. Next topic: Endocrine System: Hypothalamus, Pineal Body, Pituitary Gland, Thyroid, Adrenal Glands, Reproductive Glands.

What Are the Two Types of Immunity?

Type	Features
Innate immunity	Barriers such as skin, mucus and stomach acid, tears with lysozyme, phagocytes such as neutrophils and macrophages, natural killer cells, inflammation and fever. Acts within hours and is the same for every microbe
Adaptive immunity	Specific to an antigen, takes days at first exposure and is faster and stronger the next time because of memory cells
B cells	Mature in the bone marrow and make antibodies, which bind to antigens and mark them for destruction
T cells	Mature in the thymus. Helper T cells (CD4) coordinate the response, cytotoxic T cells kill infected cells, and regulatory T cells control the response
Antibodies	Proteins also called immunoglobulins: IgG, the most common and able to cross the placenta, IgM, IgA in milk and mucus, IgE in allergy and IgD
Primary and secondary response	The first exposure gives a slow and weak response, and a later exposure gives a quick and strong response

How Is Immunity Acquired?

Active natural immunity: from recovering from an infection.
Active artificial immunity: from a vaccine that contains a weakened or inactive microbe or its parts.
Passive natural immunity: antibodies passed from mother to baby through the placenta and breast milk, especially colostrum.
Passive artificial immunity: ready-made antibodies, such as antivenom for snake bite and antitoxin for tetanus, giving quick but short protection.
Vaccine types: live attenuated, as in measles and oral polio vaccine, inactivated, as in injectable polio and rabies, subunit and toxoid, as in hepatitis B and tetanus, viral vector and mRNA vaccines.
Herd immunity: when enough people are immune, the spread of disease is slowed and those who are not immune are indirectly protected.

What Else Should You Remember?

Lymphoid organs: the bone marrow and thymus are primary organs where cells mature, and the spleen, lymph nodes and tonsils are secondary organs where responses begin, as in Transport System: Blood Vascular System and Lymphatic System.
Allergy: an exaggerated response to harmless substances such as pollen or dust, caused by IgE and histamine, and treated by antihistamines. Anaphylaxis is a severe allergic reaction.
Autoimmune diseases: the body attacks its own tissues, as in type 1 diabetes, rheumatoid arthritis, multiple sclerosis and lupus.
AIDS: caused by HIV, a retrovirus that infects helper T cells, so that the immune system weakens, as in Viruses: Structure and Classification. Opportunistic infections then occur.
Transplant rejection: the immune system attacks tissues that differ in MHC proteins, so donors are matched and patients take drugs that suppress immunity.
Monoclonal antibodies: identical antibodies made by one cell line, developed by Georges Kohler and Cesar Milstein in 1975, used in cancer treatment and diagnosis.
Immunotherapy: treatments that use the immune system against cancer, such as checkpoint inhibitors and CAR-T cell therapy, for which India developed an indigenous product in 2023.
Immunisation in India: the Universal Immunisation Programme, started in 1985, and Mission Indradhanush aim to cover children against vaccine-preventable diseases.

Why Does It Matter for the Exam?

Concept questions: the difference between innate and adaptive immunity and between active and passive immunity.
Vaccine questions: types of vaccines and the new platforms such as mRNA.
Health policy: immunisation programmes and targets for disease elimination.
Current affairs link: vaccine research, new antibody drugs and cancer immunotherapy.
Common errors: mixing antigen and antibody, or saying that vaccines act as treatment for existing disease.

As an analytical point, immunity gives the body memory of past infections. Vaccines use this memory, which is why they are among the most effective public health tools.

Why Is This Topic Important?

Facts Cells and antibodies asked
Concept Active and passive immunity
Policy Vaccination programmes

What Came Next?

The next topic covers the endocrine system and the main glands. Next topic: Endocrine System: Hypothalamus, Pineal Body, Pituitary Gland, Thyroid, Adrenal Glands, Reproductive Glands.

Key People and Terms

Antigen Substance that triggers immunity
Antibody Protein that binds an antigen
Lymphocyte B or T white blood cell
Memory cell Cell that remembers an antigen
Vaccine Preparation that gives immunity
Autoimmunity Attack on own tissue

Exam Corner

Points to Remember

B cells make antibodies; T cells kill.
Jenner: 1796 smallpox.
HIV infects helper T cells.
IgG crosses the placenta.
Colostrum gives passive immunity.

Do Not Confuse With

Antigen and antibody Trigger versus defence protein
Active and passive immunity Own response versus ready-made antibodies
Vaccine and antivenom Prevents disease versus treats poisoning

Memory Hook

Innate in hours, adaptive with memory; B makes antibody, T kills, Jenner’s vaccine in ninety-six, HIV kills the helper.

Mains Angle

Questions ask about immunity, vaccines and immune disorders. These points add depth.

Types Innate and adaptive
Tools Vaccines and antibodies
Disorders Allergy, AIDS, autoimmunity

A Question You May Face

An original practice question, not a past paper question.

“Explain the working of the human immune system and discuss the role of vaccination in preventing disease.”

How to Answer

Types Innate, adaptive.
Cells B and T.
Vaccines Memory.
Disorders Allergy, AIDS.

GS Relevance

Prelims: types of immunity, cells and organs, antibodies, vaccines, disorders. GS Paper 3: science and technology, health.

Frequently Asked Questions

What is the difference between innate and adaptive immunity?

Innate immunity is present from birth and acts quickly and in the same way against all microbes through barriers, phagocytes, inflammation and fever. Adaptive immunity develops after exposure to a specific antigen, is carried out by B and T lymphocytes, and has memory that makes later responses faster and stronger.

How do vaccines work?

Vaccines contain a weakened or inactive microbe, a part of it such as a protein, or genetic instructions such as mRNA to make that part. They trigger the adaptive immune system to make antibodies and memory cells without causing the disease, so the body responds quickly if the real microbe enters later.

What is the difference between active and passive immunity?

In active immunity the body makes its own antibodies after infection or vaccination, which gives long protection. In passive immunity the person receives ready-made antibodies, as a baby does from the mother or a patient does from antivenom, which acts quickly but lasts only a short time.

How does HIV cause AIDS?

HIV is a retrovirus that infects and destroys helper T cells, which coordinate the immune response. As the number of these cells falls, the body can no longer fight infections and some cancers, and the person develops acquired immunodeficiency syndrome, or AIDS, with opportunistic infections such as tuberculosis.

What are autoimmune diseases?

Autoimmune diseases are conditions in which the immune system wrongly attacks the body's own tissues. Examples are type 1 diabetes, in which insulin-making cells are destroyed, rheumatoid arthritis, which damages joints, multiple sclerosis, which affects nerves, and systemic lupus erythematosus, which can affect many organs.

PYQ Practice — Statement Analysis

1 B lymphocytes produce antibodies.

True

They mature in bone marrow.

2 HIV mainly infects the red blood cells.

False

It infects helper T cells.

3 Colostrum provides passive immunity to a newborn.