L2: Immunological Principles
Complete Notes for CSIR-NET, GATE BT, DBT-BET, ICAR-JRF, ICMR-JRF & MSc Biotechnology
📚 Index / Table of Contents
1. Introduction to Immunology
Immunology is the branch of biological science concerned with the study of the immune system, immune cells, immune molecules, immune responses and the mechanisms by which organisms recognize and eliminate foreign substances. The immune system protects the body against pathogens such as bacteria, viruses, fungi and parasites, while also helping to remove damaged or abnormal cells.
The immune response can broadly be divided into two interconnected systems: innate immunity and adaptive immunity. Innate immunity provides rapid, relatively non-specific protection, whereas adaptive immunity provides highly specific responses and immunological memory.
Major components of the immune system
- Physical barriers: skin, mucosal surfaces and epithelial layers.
- Chemical barriers: stomach acid, antimicrobial peptides and enzymes.
- Innate immune cells: neutrophils, macrophages, dendritic cells, natural killer cells, mast cells and other innate immune populations.
- Adaptive immune cells: B lymphocytes and T lymphocytes.
- Soluble molecules: antibodies, complement proteins, cytokines and chemokines.
- Lymphoid organs: bone marrow, thymus, lymph nodes, spleen and mucosal lymphoid tissues.
2. Innate Immunity
Innate immunity is the first line of defense against invading microorganisms. It is present from birth and responds rapidly to common molecular features associated with pathogens or tissue damage.
Important characteristics
- It is present before exposure to a particular pathogen.
- It acts rapidly, often within minutes to hours.
- It recognizes conserved molecular patterns rather than every individual antigen.
- It includes physical, chemical, cellular and soluble defenses.
- Pattern-recognition receptors help immune cells detect microbial or damage-associated structures.
- Innate immunity activates inflammation and can influence subsequent adaptive immunity.
Physical and chemical barriers
- Skin: provides a physical barrier against entry of microorganisms.
- Mucus: traps microorganisms on mucosal surfaces.
- Ciliary movement: helps remove particles from respiratory passages.
- Gastric acid: creates an acidic environment that destroys many microorganisms.
- Lysozyme: an antimicrobial enzyme present in secretions such as tears and saliva.
- Antimicrobial peptides: can disrupt microbial membranes or interfere with microbial survival.
Major innate immune cells
- Neutrophils: important rapid responders and professional phagocytes.
- Macrophages: phagocytose pathogens and cellular debris and produce cytokines.
- Dendritic cells: important antigen-presenting cells that connect innate and adaptive immunity.
- Natural killer (NK) cells: recognize and kill certain infected or abnormal cells.
- Mast cells: contribute to inflammatory and allergic responses.
- Eosinophils: important in responses against certain parasites and in allergic inflammation.
- Basophils: circulating granulocytes involved in allergic and inflammatory responses.
Pattern recognition
Innate immune cells use pattern-recognition receptors (PRRs) to detect molecular structures associated with microbes or tissue injury. Examples include Toll-like receptors and other families of innate sensors.
Inflammation
Inflammation is an important protective response that helps recruit immune cells and soluble mediators to sites of infection or tissue injury. Classical features of inflammation include redness, heat, swelling, pain and loss of function, although the exact presentation varies with the tissue and cause.
- Vascular changes increase delivery of plasma proteins and immune cells.
- Chemokines help recruit leukocytes.
- Cytokines coordinate communication between immune and tissue cells.
- Phagocytes can engulf and destroy microorganisms.
- Complement can contribute to opsonization, inflammation and microbial killing.
3. Adaptive Immunity
Adaptive immunity is a highly specific branch of immunity mediated primarily by B and T lymphocytes. It develops after exposure to antigens and has the ability to generate immunological memory.
Major features
- Specificity: immune receptors can distinguish different antigens.
- Diversity: enormous numbers of antigen receptor specificities can be generated.
- Clonal expansion: antigen-specific lymphocytes proliferate after activation.
- Memory: memory B and T cells can persist after an immune response.
- Self-tolerance: mechanisms normally prevent strong responses against healthy self tissues.
B lymphocytes
B cells are central to humoral immunity. After appropriate activation, B cells can differentiate into plasma cells that secrete antibodies. Some activated B cells become memory B cells.
- B cells carry B-cell receptors on their surface.
- Activated B cells can undergo clonal expansion.
- Plasma cells produce antibodies.
- Memory B cells contribute to rapid responses during subsequent exposure.
T lymphocytes
T cells are central to cell-mediated immunity. Different T-cell subsets have different functions.
- Helper T cells: coordinate immune responses through cytokines and cell-cell interactions.
- Cytotoxic T cells: can kill infected or abnormal cells.
- Regulatory T cells: help control excessive immune responses and maintain tolerance.
- Memory T cells: provide long-term immunological memory.
4. Innate Immunity vs Adaptive Immunity
| Feature | Innate Immunity | Adaptive Immunity |
|---|---|---|
| Onset | Rapid | Usually slower during primary exposure |
| Specificity | Broad recognition of conserved patterns | Highly antigen-specific |
| Main cells | Neutrophils, macrophages, NK cells, dendritic cells and others | B cells and T cells |
| Memory | No classical antigen-specific memory comparable to adaptive memory | Strong immunological memory |
| Receptors | Germline-encoded pattern-recognition receptors | Highly diverse BCRs and TCRs |
| Major functions | Barrier defense, inflammation, phagocytosis and early pathogen control | Antibody production, T-cell responses and long-term protection |
Innate = rapid + broad + first line
Adaptive = specific + memory + B/T lymphocytes
5. Active and Passive Immunity
Immunity can also be classified according to how protective immunity is acquired. The two major categories are active immunity and passive immunity.
Active immunity
Active immunity develops when the individual's own immune system responds to an antigen. The response may occur naturally following infection or artificially following vaccination.
- The person's own immune system produces the response.
- Antibodies and/or antigen-specific lymphocytes are generated.
- Memory cells can develop.
- Protection usually develops more slowly than passive immunity.
- Protection can be relatively long-lasting.
Natural active immunity
Natural active immunity occurs after natural exposure to an infectious agent. The immune system responds to microbial antigens and may generate memory.
Artificial active immunity
Artificial active immunity is generated through vaccination. A vaccine presents an antigen or antigenic information in a manner designed to stimulate protective immunity without causing the targeted disease in the usual way.
Passive immunity
Passive immunity occurs when preformed antibodies are transferred to an individual rather than being generated by that individual's immune response.
- Protection is generally rapid.
- It does not require the recipient to mount a primary antibody response.
- Protection is generally temporary.
- Classical passive antibody transfer does not create the same immunological memory as active immunization.
Natural passive immunity
- Maternal antibodies can be transferred to the fetus through the placenta.
- Antibodies can also be transferred through breast milk, particularly secretory IgA.
Artificial passive immunity
Artificial passive immunity involves administration of antibody-containing preparations. Such preparations can provide immediate antibody-mediated protection when rapid protection is required.
| Feature | Active Immunity | Passive Immunity |
|---|---|---|
| Source | Own immune response | Transferred antibodies |
| Onset | Usually slower | Rapid |
| Memory | Can develop | Not generated by the transferred antibodies themselves |
| Duration | Often longer-lasting | Usually temporary |
| Examples | Infection or vaccination | Maternal antibodies or antibody preparations |
6. Antibodies and Humoral Immunity
Antibodies, also called immunoglobulins, are antigen-binding proteins produced by B-cell-derived plasma cells. They recognize specific epitopes on antigens.
Basic antibody structure
A typical antibody molecule contains two heavy chains and two light chains. Disulfide bonds hold the chains together. The variable regions participate in antigen recognition, whereas constant regions contribute to the biological functions of different antibody classes.
- Fab region: responsible primarily for antigen binding.
- Fc region: interacts with Fc receptors and other components of the immune system.
- Variable region: determines antigen-binding specificity.
- Constant region: determines many effector properties.
Major immunoglobulin classes
| Immunoglobulin | Important characteristics |
|---|---|
| IgG | Major serum antibody; important in systemic immunity; can cross the placenta. |
| IgM | Often the first major antibody class produced in a primary response; commonly forms pentamers in secreted form. |
| IgA | Important at mucosal surfaces; secretory IgA is prominent in mucosal secretions. |
| IgE | Important in allergic responses and defense against certain parasites. |
| IgD | Found mainly as a B-cell surface immunoglobulin and participates in B-cell biology. |
7. Monoclonal Antibodies
A monoclonal antibody preparation contains antibodies derived from a single B-cell clone and therefore recognizes a particular antigenic determinant or epitope with defined specificity.
Polyclonal vs monoclonal antibodies
| Feature | Polyclonal antibodies | Monoclonal antibodies |
|---|---|---|
| Origin | Multiple B-cell clones | Single B-cell clone |
| Epitope recognition | Usually multiple epitopes | Usually one defined epitope |
| Specificity | Mixed antibody population | Highly specific |
| Batch consistency | Can vary between preparations | High consistency when produced under controlled conditions |
| Applications | Research, detection and some diagnostic uses | Diagnostics, research and therapeutic applications |
Hybridoma technology
The classical method for producing monoclonal antibodies is hybridoma technology. The central idea is to combine the antibody-producing capacity of a B cell with the long-term growth capability of a suitable myeloma cell.
- An animal is immunized with the antigen of interest.
- Antibody-producing B lymphocytes are obtained from an appropriate lymphoid tissue.
- The B cells are fused with suitable myeloma cells.
- The resulting hybrid cells are called hybridomas.
- Hybridomas are selected using an appropriate selection system.
- Individual hybridoma clones are isolated.
- Clones are screened for production of the desired antibody.
- A positive clone can be expanded for antibody production.
Applications of monoclonal antibodies
- Detection of specific proteins.
- Diagnostic assays.
- Identification of cell-surface markers.
- Research involving signaling pathways.
- Immunohistochemistry.
- Flow cytometry.
- Therapeutic targeting of specific molecules or cells.
8. Vaccines and Immunization
A vaccine is designed to stimulate an immune response against a particular pathogen or antigen so that subsequent exposure can be controlled more effectively.
Basic principle of vaccination
- A vaccine introduces an antigen, antigenic component or antigen-related information.
- Antigen-presenting cells and lymphocytes participate in the immune response.
- B and/or T lymphocytes can become activated.
- Antibody-producing cells can develop.
- Memory B and T cells can be generated.
- Subsequent exposure can therefore produce a faster and stronger response.
Major vaccine categories
1. Live attenuated vaccines
- Contain a weakened form of a microorganism.
- Can induce strong immune responses.
- Often generate both cellular and humoral components.
- Appropriate use depends on the specific vaccine and recipient.
2. Inactivated vaccines
- Contain microorganisms that have been inactivated.
- Cannot replicate in the recipient.
- May require multiple doses or boosters for optimal protection.
3. Subunit vaccines
- Contain selected antigenic components rather than the complete pathogen.
- Can have good safety profiles because they contain limited pathogen components.
- Adjuvants may be used to improve immune stimulation.
4. Toxoid vaccines
Toxoid vaccines use an inactivated toxin or toxoid to induce immunity against the harmful effects of a toxin.
5. Conjugate vaccines
A poorly immunogenic polysaccharide antigen can be linked to a protein carrier to improve immune recognition and generate a more effective response, particularly in young children.
6. Nucleic-acid-based and recombinant approaches
Modern vaccine platforms can use recombinant proteins, viral vectors or nucleic acid approaches to deliver antigenic information. The exact mechanism differs between platforms, but the goal is to generate protective adaptive immunity.
Primary and secondary immune responses
During a primary immune response, antigen-specific lymphocytes are activated and expanded. Memory cells are generated. During subsequent exposure to the same antigen, memory cells can respond more rapidly and effectively.
9. Immunodiagnostic Techniques
Immunodiagnostics use antigen-antibody interactions for the detection or measurement of biological molecules. These techniques are widely used in clinical diagnosis, biotechnology, research and laboratory medicine.
Antigen-antibody interaction
The fundamental principle behind many immunodiagnostic assays is the specific interaction between an antibody and its antigen. The strength and specificity of this interaction depend on molecular complementarity and non-covalent interactions such as hydrogen bonds, electrostatic interactions, hydrophobic interactions and van der Waals forces.
Important immunodiagnostic techniques
- ELISA
- Western blotting
- Immunofluorescence
- Immunohistochemistry
- Rapid immunochromatographic assays
- Agglutination assays
- Precipitation assays
- Flow cytometry using antibody-based detection
- Radioimmunoassay
ELISA
ELISA stands for Enzyme-Linked Immunosorbent Assay. It is one of the most important immunological assays used to detect or quantify antigens or antibodies.
The assay uses an enzyme-linked detection system. When the appropriate substrate is added, the enzyme produces a measurable signal, often a color change. The intensity of the signal can be related to the amount of target depending on the assay design.
Major types of ELISA
Direct ELISA
- Antigen is immobilized on a solid surface.
- An enzyme-labeled primary antibody binds directly to the antigen.
- Substrate produces a measurable signal.
Indirect ELISA
- Antigen is immobilized.
- Primary antibody from the sample binds to the antigen.
- An enzyme-linked secondary antibody recognizes the primary antibody.
- Substrate produces the signal.
Sandwich ELISA
- A capture antibody is immobilized.
- The target antigen binds to the capture antibody.
- A detection antibody binds another epitope of the antigen.
- The detection system generates a measurable signal.
Competitive ELISA
- Sample analyte competes with a labeled or reference analyte for antibody binding.
- The signal is often inversely related to the concentration of target, depending on assay design.
| ELISA Type | Key Principle | Important Exam Point |
|---|---|---|
| Direct | Enzyme-labeled primary antibody | Fewer antibody steps |
| Indirect | Primary antibody + labeled secondary antibody | Useful for antibody detection |
| Sandwich | Capture + antigen + detection antibody | High specificity |
| Competitive | Competition between analytes | Signal often decreases as target increases |
Western blotting
Western blotting is used to detect a particular protein within a mixture. Proteins are separated by electrophoresis, transferred to a membrane and then detected using antibodies.
- Protein extraction.
- Protein separation by SDS-PAGE.
- Transfer to membrane.
- Blocking of non-specific binding sites.
- Primary antibody incubation.
- Secondary antibody incubation when applicable.
- Signal detection.
Immunofluorescence
Immunofluorescence uses antibodies associated with fluorescent labels to visualize specific antigens in cells or tissues.
- Direct immunofluorescence uses a labeled primary antibody.
- Indirect immunofluorescence uses an unlabeled primary antibody and a labeled secondary antibody.
- Fluorescence microscopy is used for visualization.
Agglutination
Agglutination is the visible clumping of particulate antigens caused by antibody-mediated cross-linking. It can be used in various diagnostic assays.
Precipitation
Precipitation occurs when soluble antigen and antibody form sufficiently large immune complexes that become insoluble under appropriate conditions.
Flow cytometry
Flow cytometry measures characteristics of individual cells as they pass through a laser beam. Fluorescently labeled antibodies can be used to detect specific cell-surface or intracellular markers.
- Cells are passed individually through the interrogation point.
- Laser light interacts with cells and fluorophores.
- Detectors measure light scattering and fluorescence.
- Different cell populations can be identified using marker combinations.
10. High-Yield Exam Points
🔥 Must Remember
- Innate immunity is the rapid first-line defense.
- Adaptive immunity is highly specific and develops immunological memory.
- B cells are central to humoral immunity.
- T cells are central to cell-mediated immunity.
- Plasma cells are major antibody-secreting cells.
- Vaccination generally induces active immunity.
- Transfer of preformed antibodies produces passive immunity.
- IgG is the major immunoglobulin in serum and can cross the placenta.
- IgM is prominent in the primary immune response.
- IgA is particularly important at mucosal surfaces.
- IgE is associated with allergy and responses against certain parasites.
- Hybridoma technology is classically used for monoclonal antibody production.
- Monoclonal antibodies originate from a single B-cell clone.
- ELISA uses enzyme-linked detection.
- Sandwich ELISA uses capture and detection antibodies.
- Western blot detects specific proteins after electrophoretic separation and membrane transfer.
- Flow cytometry can identify cell populations using fluorescent antibodies.
- Immunofluorescence uses fluorescent labels for antigen localization.
Common conceptual traps
- Active does not mean "stronger." It means the individual's own immune system generated the response.
- Passive does not mean "weak." Passive antibodies can provide rapid protection but are generally temporary.
- Monoclonal does not mean one antibody molecule. It means the antibody population is derived from a single clone.
- ELISA is not limited to detecting antigens. Different formats can be designed to detect antibodies or antigens.
- Adaptive immunity is not completely independent of innate immunity. Innate immune mechanisms help initiate and shape adaptive responses.
11. Quick Revision Table
| Topic | One-Line Revision |
|---|---|
| Innate immunity | Rapid, broad first-line defense. |
| Adaptive immunity | Specific immunity with immunological memory. |
| B cell | Major cell of humoral immunity. |
| Plasma cell | Produces and secretes antibodies. |
| T cell | Major cell type involved in cell-mediated immunity. |
| Active immunity | Generated by the individual's own immune response. |
| Passive immunity | Protection obtained through transferred antibodies. |
| IgG | Major serum immunoglobulin; crosses placenta. |
| IgM | Prominent early antibody response. |
| IgA | Important mucosal immunoglobulin. |
| IgE | Allergy and parasite-associated immunity. |
| Monoclonal antibody | Antibody population derived from one clone. |
| Hybridoma | Classically combines antibody-producing B cell with immortal growth capability. |
| ELISA | Enzyme-linked immunological detection assay. |
| Western blot | Antibody-based detection of specific proteins. |
| Flow cytometry | Cell-by-cell analysis using light scattering and fluorescence. |
12. Practice MCQs – Immunological Principles
Instructions: Select one option for each question and click Submit Test. The score and correct answers will be shown only after submission.
13. One-Minute Revision
- Innate: rapid, broad, first-line defense.
- Adaptive: specific, diverse, memory-forming.
- B cells: humoral immunity and antibody production.
- T cells: cell-mediated immunity.
- Active immunity: your immune system makes the response.
- Passive immunity: antibodies are received from another source.
- IgG: major serum antibody and crosses placenta.
- IgM: important early antibody response.
- IgA: mucosal immunity.
- IgE: allergy and parasite-associated responses.
- Monoclonal antibody: derived from one B-cell clone.
- Hybridoma: antibody-producing B cell + immortal growth partner.
- Vaccine: stimulates active adaptive immunity.
- ELISA: enzyme-linked immunological assay.
- Sandwich ELISA: capture antibody + antigen + detection antibody.
- Western blot: detection of specific proteins.
- Flow cytometry: cell-by-cell analysis using light and fluorescence.
INNATE = Immediate
ADAPTIVE = Antigen-specific + Memory
B CELL = Antibody
T CELL = Cell-mediated response
VACCINE = Active immunity
ANTIBODY TRANSFER = Passive immunity
HYBRIDOMA = Monoclonal antibody production
ELISA = Enzyme-linked detection
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