The body's defense system — from skin and stomach acid (innate) to antibodies, T cells, and memory (acquired) — plus the related pathologies of allergy, autoimmunity, AIDS, and cancer that NEET tests every year.
Immunology Why this chapter matters Every day your body is bombarded by bacteria, viruses, fungi, parasites, allergens, and the occasional rogue cell of your own that has begun to divide out of control. The fact that you stay healthy most of the time is the work of a beautifully layered defense system called the immune system . NEET tests this chapter heavily — between innate barriers, antibody isotypes, T cell recognition, AIDS, and cancer, immunology contributes a reliable cluster of marks every year. This unit moves in three arcs. First, the two-tier defense system: innate immunity (immediate, non-specific) and acquired immunity (slow, specific, remembered). Second, how the immune system can fail — by overreacting (allergy), by attacking self (autoimmunity), or by being attacked itself (AIDS). Third, cancer: the molecular accident of uncontrolled cell division, and how the immune system is itself a tumour-monitoring service. Master map of immunity — innate (skin, mucus, phagocytes, complement, NK cells) on the left, acquired (B cells, antibodies, T cells, memory) on the right. This single diagram organises the entire chapter. The two arms of immunity Innate immunity is what you are born with. It is non-specific (treats every microbe the same way), immediate (acts within minutes), and has no memory (a second exposure produces the same response as the first). Acquired immunity is learned. It is highly specific (recognises one antigen out of millions), slow to develop on first exposure (5–10 days), and crucially has memory — a second encounter produces a faster, stronger response. Vaccination exploits this. Innate Immunity Non-specific, immediate defense present from birth. Includes physical barriers (skin), physiological barriers (stomach acid), cellular barriers (phagocytes, NK cells), and cytokine/complement responses. Pathogen-specific defense that develops after exposure to an antigen. Characterised by specificity, diversity, memory, and the ability to distinguish self from non-self. Mediated by B and T lymphocytes. Acquired Immunity Specificity Non-specific — same response to every pathogen Highly specific — one antibody/TCR per antigen Speed Minutes to hours 5–10 days on first exposure Memory Absent Present — basis of vaccination Cells involved Macrophages, neutrophils, NK cells B lymphocytes, T lymphocytes Components Skin, mucus, lysozyme, complement, phagocytes Antibodies, helper T cells, cytotoxic T cells Innate Immunity Acquired Immunity Attribute Innate = Immediate; Acquired = Adaptive. Innate vs Acquired Immunity Innate immunity: four lines of defense Innate immunity is conventionally divided into four barriers that work in series. A pathogen has to breach each one to cause disease, and most never make it past the first. 1. Physical barriers The skin is the body's largest organ and the single most important physical barrier. Its outer layer of dead, keratinised cells is impermeable to most microbes. Mucous membranes line the respiratory, gastrointestinal, and urogenital tracts. Their sticky mucus traps microbes, and in the airways the mucociliary escalator — a coordinated sweeping motion of cilia — moves trapped particles up and out of the lungs to be swallowed or expelled. 2. Physiological barriers The low pH of the stomach ( 1.5 - 3.5 ) is bactericidal — most ingested microbes are killed before they reach the intestine. Lysozyme , an enzyme present in tears, saliva, and nasal secretions, hydrolyses the peptidoglycan layer of bacterial cell walls and is particularly effective against Gram-positive bacteria. Bile salts in the gut and the slightly acidic pH of vaginal secretions add further layers. Lysozyme A hydrolytic enzyme present in tears, saliva, and other body secretions that breaks down the peptidoglycan layer of bacterial cell walls, especially effective against Gram-positive bacteria. neet-alert NEET alert: The pH of gastric juice ( 1.5 - 3.5 ) is a physiological barrier, not a physical one. NEET routinely tests this distinction — pH is chemistry, the skin is structure. 3. Cellular barriers Several white blood cell types patrol the body looking for invaders. Neutrophils and macrophages are professional phagocytes — they engulf and digest microbes via phagocytosis . Macrophages additionally act as Antigen Presenting Cells (APCs), linking innate and acquired immunity. Natural Killer (NK) cells are lymphocytes that target virally-infected and tumour cells by spotting those that have downregulated their MHC class I molecules — a common viral evasion trick. NK cells then release perforin and granzymes to punch holes and trigger apoptosis. Engulfment and digestion of foreign particles by phagocytic cells such as macrophages and neutrophils. The particle is taken into a phagosome, which fuses with a lysosome to form a phagolysosome. Phagocytosis Antigen Presenting Cells (APCs) Cells like macrophages and dendritic cells that engulf pathogens and display fragments of their antigens on MHC molecules to T cells, linking the innate system to the acquired system. 4. Cytokine and complement barriers When cells are infected they release interferons — cytokines that warn neighbouring cells and induce an antiviral state. The complement system is a cascade of about thirty plasma proteins (the most studied are C1 - C9 ) that, once activated, produce three outcomes: opsonisation (coating pathogens to make them tastier to phagocytes), inflammation (recruiting more immune cells), and lysis (assembling the Membrane Attack Complex , MAC , which punctures the microbe's membrane). Complement System A cascade of plasma proteins ( C1 - C9 ) that enhances immunity via three pathways — Classical, Lectin, and Alternative. Outcomes include opsonisation, inflammation, and lysis through formation of the Membrane Attack Complex ( MAC ). Three pathways of complement activation Classical Pathway — triggered when IgG or IgM antibodies bind to an antigen on a pathogen. Linked to acquired immunity. Lectin Pathway — triggered when mannose-binding lectin in plasma binds to specific sugar patterns on microbial surfaces. Antibody-independent. Alternative Pathway — spontaneously activated by bacterial cell wall components ( LPS ). Antibody-independent and the fastest of the three; pure innate immunity. The complement system is only part of acquired immunity. The Alternative and Lectin pathways are antibody-independent and provide immediate innate defense. Only the Classical pathway requires antibodies — the complement system bridges innate and acquired immunity. Physical Skin (keratin), mucous membranes, cilia Most microbes — physical exclusion Physiological Stomach acid (pH 1.5–3.5), lysozyme in tears/saliva, bile Bacteria — chemical destruction Cellular Neutrophils, macrophages, NK cells Engulf or kill any that breach the surface Cytokine / Complement Interferons, C1–C9, MAC Viruses and bacteria — chemical warning and lysis Examples Target Physical → Physiological → Cellular → Cytokine. Four P/C words. Barrier type Four lines of innate defense Acquired immunity: humoral and cell-mediated If a pathogen survives innate defenses, acquired immunity takes over. It has two arms. Humoral immunity (also called antibody-mediated immunity, AMI) is run by B lymphocytes that mature in the bone marrow and secrete soluble antibodies. Cell-mediated immunity (CMI) is run by T lymphocytes that mature in the thymus and act through direct cell-to-cell contact. The two arms are not isolated — helper T cells are required to activate most B cell responses. The classic IgG molecule — two heavy chains and two light chains held by disulfide bonds in an H 2 L 2 arrangement, with two antigen-binding sites at the tips of the Y. Each antibody (immunoglobulin) molecule is a Y-shaped protein built from four polypeptide chains: two heavy (H) chains and two light (L) chains , written as H 2 L 2 . The tips of the Y are the antigen-binding sites (variable regions), and the stem (Fc region) determines what the antibody does once it has bound — fix complement, cross the placenta, attach to mast cells, and so on. Disulfide bonds hold the chains together. The five immunoglobulin classes Immunoglobulin Classes Five antibody isotypes distinguished by their heavy chain: IgG (monomer, most abundant, crosses placenta, secondary response); IgM (pentamer, first to appear, primary response); IgA (dimer in secretions); IgE (allergy and parasite defense); IgD (B cell surface receptor). IgG Monomer Most abundant in serum; only Ig to cross placenta Secondary response; passive immunity to fetus Crosses placenta IgM Pentamer Blood, B cell surface First antibody produced in primary response; agglutinator First responder IgA Dimer Mucosal secretions — saliva, tears, milk, gut Mucosal defense; in colostrum protects newborn Secretions IgE Monomer Bound to mast cells and basophils Allergy (Type I hypersensitivity); helminth defense Allergy IgD Monomer Surface of mature B cells B cell antigen receptor; role still being defined B cell receptor Antibody isotypes — structure, location, function Structure Where found Key function NEET hook Isotype GAMED: IgG, IgA, IgM, IgE, IgD. Side-by-side comparison of the five antibody structures. Clean labeled diagram of IgG (Y), IgM (pentamer with J chain), IgA (dimer with secretory component), IgE (Y bound to mast cell), IgD (Y on B cell surface). Same scale, same colour for heavy chains, lighter colour for light chains. NEET alert: IgG is the only antibody that crosses the placenta. IgA is the most abundant antibody in secretions (not serum — that's IgG). Don't mix these two. neet-alert Primary vs secondary response On first exposure to an antigen, B cells take 5–10 days to mount an antibody response. The antibodies are mostly IgM and the peak titer is modest. Crucially, some activated B cells become memory B cells and persist for years. On second exposure to the same antigen, these memory cells respond within hours, produce mostly IgG , and reach a much higher titer. This is the principle exploited by every vaccine ever made — from Edward Jenner's smallpox to Louis Pasteur's anthrax to modern mRNA vaccines. Primary vs secondary immune response Primary response Secondary response Feature First = slow, IgM. Second = fast, IgG. Lag phase 5–10 days 1–4 days Peak titer Low High (often 100× primary) Dominant antibody IgM IgG Duration Short — antibodies decline rapidly Long — antibodies persist much longer Cells involved Naive B cells differentiating to plasma cells Pre-existing memory B cells reactivating remember Remember: Primary response is dominated by IgM , secondary by IgG . Vaccines work by safely generating a primary response so that natural infection encounters a fast, IgG -dominated secondary response. Cell-mediated immunity: T cells and MHC T cells cannot recognise free antigens floating in plasma — antigens have to be presented on the surface of another cell, bound to a Major Histocompatibility Complex ( MHC ) molecule. There are two MHC classes and two corresponding T cell subsets, and the pairing is one of the most frequently tested facts in NEET immunology. Helper T cells ( CD4 + ) recognise antigen presented on MHC class II (which is found only on APCs — macrophages, dendritic cells, B cells). They are the 'generals' — they release cytokines to activate B cells and cytotoxic T cells. Cytotoxic T lymphocytes ( CD8 + , CTLs) recognise antigen presented on MHC class I (found on virtually every nucleated body cell). They kill the displaying cell using perforin and granzymes — the same machinery NK cells use. Memory T cells of both subsets persist after infection and provide rapid response on re-exposure, the cell-mediated counterpart of memory B cells. Two T cell subsets, two MHC partners neet-alert NEET alert — memorise the pairing: CD4 + MHC II and CD8 + MHC I . Easy mnemonic: 4 2 = 8 1 . Active vs passive immunity When the body's own immune system makes antibodies in response to an antigen, the protection is called active immunity . It is slow to develop but long-lasting because of memory cells. Passive immunity is the transfer of pre-formed antibodies from one individual to another. It is immediate but temporary — it lasts only as long as the donor antibodies survive (weeks), because no memory cells are produced. Source of antibodies Self — body makes them on antigen exposure External — pre-formed antibodies given Onset Slow (5–10 days) Immediate Duration Long-lasting (memory cells) Short — weeks at most Natural example Surviving an infection (e.g., chickenpox) IgG across placenta; IgA in colostrum Artificial example Vaccination Anti-venom injection; anti-tetanus serum Active immunity Passive immunity Active = you make it. Passive = you receive it. Feature Active vs passive immunity When the immune system overreacts: allergies Sometimes the immune system mounts a strong response to a substance that is not actually harmful — pollen, dust mite faeces, peanut proteins. This is Type I hypersensitivity , commonly called allergy. On first exposure to the allergen, the body produces IgE antibodies, which coat the surface of mast cells and basophils. On second exposure, the allergen cross-links these IgE molecules, triggering degranulation — mast cells dump preformed histamine and other inflammatory mediators into the tissue, producing the familiar symptoms of rhinitis, asthma, hives, or in severe cases anaphylaxis. An exaggerated immune response (Type I hypersensitivity) to a harmless substance (allergen), mediated by IgE bound to mast cells. Triggers release of histamine and other inflammatory mediators. Allergy Pollen, dust mites, animal dander — allergic rhinitis ('hay fever'), sneezing, itchy eyes. Specific foods — peanuts, shellfish, eggs — can cause urticaria or anaphylaxis. Drugs — penicillin is a classic example; can be life-threatening. Insect venom — bee and wasp stings can trigger anaphylactic shock. Common allergens and typical symptoms Histamine is preformed and stored in mast cell granules. The trigger is allergen cross-linking IgE already bound to the mast cell surface — not new antibody production. Antihistamines and steroids work by blocking these mediators. Allergy is triggered by histamine production from B cells. When the immune system attacks self: autoimmunity The immune system normally distinguishes 'self' from 'non-self' through a process called self-tolerance , established during T cell maturation in the thymus. When this tolerance breaks down, the body produces antibodies (autoantibodies) or T cells that attack its own tissues — an autoimmune disease . NCERT names rheumatoid arthritis as the classic example, where the immune system attacks joint linings. Other major autoimmune diseases include systemic lupus erythematosus (SLE — antibodies against nuclear components), Type 1 diabetes (T cell destruction of pancreatic -cells), and multiple sclerosis . Autoimmunity A condition where the immune system mistakenly attacks the body's own cells and tissues due to a breakdown of self-tolerance. Examples: rheumatoid arthritis, SLE, Type 1 diabetes. All diseases are caused by external pathogens. Many diseases — autoimmune disorders, genetic defects, cancers, allergies — are caused by malfunction of the body's own systems, not by an outside microbe. AIDS and HIV Acquired Immunodeficiency Syndrome (AIDS) is caused by the Human Immunodeficiency Virus (HIV), a retrovirus of the lentivirus family. HIV's genome is RNA, but the virus carries an enzyme called reverse transcriptase that copies its RNA into DNA after entering a host cell. This viral DNA is then integrated into the host genome by another viral enzyme, integrase , where it can lie dormant or actively produce new virus particles. HIV particle anatomy — RNA genome, reverse transcriptase, capsid, envelope with gp120 and gp41 spikes — plus the full life cycle from receptor binding to budding. Human Immunodeficiency Virus; an enveloped RNA retrovirus that specifically targets CD4 + T helper cells. Uses reverse transcriptase to convert RNA to DNA, which is then integrated into host genome by integrase. HIV Enzyme-Linked Immunosorbent Assay; a sensitive antibody-based test used to diagnose HIV infection by detecting anti-HIV antibodies in the patient's serum. ELISA A tissue-resident immune cell that stores preformed histamine and other inflammatory mediators in granules. Cross-linking of surface-bound IgE by allergen triggers degranulation and Type I hypersensitivity. Mast Cell Why HIV is so devastating HIV's surface glycoprotein gp120 binds the CD4 molecule on helper T cells — the very cells that orchestrate the entire acquired immune response. As helper T cells are progressively destroyed, the patient loses the ability to mount effective immune responses. The hallmark of full-blown AIDS is therefore not HIV itself but a collapse of immunity that allows opportunistic infections ( Pneumocystis pneumonia, oral thrush, tuberculosis) and unusual cancers (Kaposi's sarcoma) to take hold. Step-by-step HIV replication inside a CD4 + helper T cell — viral binding, fusion, reverse transcription, integration into host DNA, transcription, assembly, and budding. HIV replication cycle (NEET-frequent) Attachment — gp120 binds CD4 receptor (and a co-receptor) on a helper T cell. Entry — viral envelope fuses with the cell membrane; RNA and enzymes enter the cytoplasm. Reverse transcription — reverse transcriptase copies viral RNA into double-stranded DNA. Integration — viral DNA is inserted into the host chromosome by integrase; the cell now carries the virus permanently. Transcription and translation — host machinery produces new viral RNA and proteins. Assembly and budding — new virus particles assemble and bud off the cell, often killing it. NEET alert: HIV is diagnosed by ELISA (Enzyme-Linked Immunosorbent Assay), which detects antibodies against HIV in the patient's serum. Confirmation uses Western blot or PCR. neet-alert HIV transmission routes (NCERT-listed) Sexual contact with an infected person. Sharing infected needles (especially among drug users). Transfusion of contaminated blood or blood products. From an infected mother to her fetus through the placenta. Cancer: when cells stop listening to the brakes Cancer is fundamentally a disease of uncontrolled cell division . A normal cell divides only when signalled to, stops when it should, and self-destructs (apoptosis) if its DNA is damaged beyond repair. A cancer cell has lost these controls. The result is a tumour — a mass of cells that does not respect the boundaries and rules of the tissue it came from. Benign tumour on the left — encapsulated, localised, harmless. Malignant tumour on the right — invasive, metastasising through blood vessels. The single defining difference is the ability to spread. Benign vs malignant tumours Tumours come in two flavours. A benign tumour stays put — it grows locally, is usually encapsulated, and does not invade surrounding tissue. A malignant tumour is what we call cancer in everyday language. It grows rapidly, invades surrounding tissue, and crucially can metastasise — break off, travel through blood or lymph, and seed secondary tumours in distant organs. Metastasis is the single most dangerous property of malignant tumours and the cause of most cancer deaths. The spread of malignant cells from a primary tumour to distant organs via blood or lymphatic vessels, where they establish secondary tumours. Metastasis is the defining feature of malignancy and the main cause of cancer mortality. Metastasis Benign vs malignant tumours Property B = Boundaries respected; M = Metastasises. Benign Malignant Growth rate Slow, often stops at a certain size Rapid and unrestrained Capsule Usually encapsulated Non-encapsulated, invasive Invasion of nearby tissue Absent Present Metastasis Absent Present — defining feature Example Fibroma, lipoma Carcinoma, sarcoma, leukaemia Histology side-by-side: well-encapsulated benign tumour vs invasive malignant tumour breaching tissue boundaries. A microscope-style two-panel image. Left panel: a benign tumour with a clear fibrous capsule, regular cells, low mitotic figures. Right panel: a malignant tumour with irregular invasive borders, pleomorphic cells, abundant mitotic figures, and a vessel containing tumour emboli. All tumours are cancer. Only malignant tumours are cancer. Benign tumours can be uncomfortable or even dangerous due to location (e.g., in the brain) but do not invade or metastasise. The molecular basis: oncogenes and tumour suppressors Two classes of genes regulate the cell cycle. Proto-oncogenes are normal genes that drive cell division when needed — they are the accelerators. When mutated or overexpressed, they become oncogenes that drive cell division relentlessly. The classic NEET example is the Ras oncogene, mutated in many human cancers. Tumour suppressor genes are the brakes. They detect DNA damage and halt the cell cycle or trigger apoptosis. The most famous tumour suppressor is p53 , called the 'guardian of the genome'. Loss of p53 function removes the brake, allowing damaged cells to survive and divide. Oncogenes Mutated forms of normal proto-oncogenes that promote uncontrolled cell proliferation. Example: Ras . They function as 'accelerators' stuck in the on position. Tumour Suppressor Genes Genes such as p53 that normally halt the cell cycle or induce apoptosis in response to DNA damage, preventing cancer. Loss of their function contributes to malignancy. remember Remember: Cancer = oncogene activation (accelerator stuck on) + tumour suppressor inactivation (brake failed). Both kinds of damage usually accumulate over years. Major carcinogens (NCERT) Physical carcinogens — ionising radiation (X-rays, gamma rays), ultraviolet radiation from the sun. Chemical carcinogens — tobacco smoke (a major cause of lung cancer), industrial chemicals like benzene. Biological carcinogens — oncogenic viruses such as HPV (cervical cancer), HBV (liver cancer), EBV (lymphoma). Cancer is detected by biopsy (microscopic examination of a tissue sample), histopathology , blood tests for tumour markers, and imaging techniques like MRI , CT scan , and radiography . Treatment combines surgery (remove the tumour), radiation therapy (kill localised cells), and chemotherapy (drugs that target rapidly dividing cells). -interferon is used in some cases to boost the patient's own immune response against the tumour. GAMED = the five antibody classes in order of relative abundance and importance: IgG (most abundant), IgA (secretions), IgM (first responder), IgE (allergy), IgD (B cell receptor). 4 2 = 8 1 — Helper T cells are CD4 + and recognise MHC II ; cytotoxic T cells are CD8 + and recognise MHC I . Just remember the equation. Complement pathways = CLA — C lassical (antibody), L ectin (mannose), A lternative (spontaneous on bacterial surfaces).