Antibody Structure & Function: Immune Response Explained

Monoclonal antibodies are produced from a single cloned cell, all identical and binding to only one specific antigen. Production: inject mouse with antigen; spleen lymphocytes produce specific antibodies; fuse with tumor cells to form hybridoma cells; hybridomas divide rapidly to produce millions of identical monoclonal antibodies. Applications: diagnostic tests (pregnancy testing, identifying molecules with fluorescent markers); cancer treatment (delivering radioactive therapy directly to cancer cells, minimizing harm to healthy tissue). Pros: produced quickly, bind to almost any substance, enable rapid diagnosis, target cancer treatments. Cons: expensive, cause unexpected side effects, time-consuming, ethical objections to animal use.

Monoclonal antibodies (mAbs) are highly specific therapeutic agents derived from human or modified mouse sources that offer significant advantages over traditional treatments due to their targeted mechanism of action; they are classified into four types based on their origin—murine (suffix -omab), chimeric (suffix -ximab), humanized (suffix -zumab), and fully human (suffix -umab)—and are used across diverse medical applications including autoimmune disease management (e.g., adalimumab targeting TNF for rheumatoid arthritis), antiplatelet therapy (e.g., abciximab preventing blood clotting), asthma treatment (omalizumab inhibiting allergic inflammation), and cancer therapy through multiple mechanisms such as antibody-dependent cell-mediated cytotoxicity (rituximab targeting CD20), growth signal blocking (trastuzumab inhibiting EGFR), angiogenesis prevention (bevacizumab blocking VEGF), and targeted radiation delivery (ibritumomab conjugated with radioisotopes).

Oncology is the most profitable and fastest-growing sector for monoclonal antibodies. Keytruda (pembrolizumab) became the number one drug in terms of revenue in 2024 with $29.5 billion sales, and reached $15.5 billion in the first half of 2025 alone. It has become the standard therapy for many tumors including melanoma, lung cancer, cervical cancer, and colorectal cancer. The drug represents a paradigm shift in cancer treatment through targeted immune checkpoint inhibition.

Monoclonal antibodies are identical antibodies produced by a single B cell clone, derived from hybridoma technology developed by George Köhler and César Milstein (Nobel Prize 1984), which can be made in large quantities and serve as the cornerstone of immunology; they differ from polyclonal antibodies in specificity, cost, and cross-reactivity, and are classified into naked (e.g., Trastuzumab for breast cancer, Rituximab for lymphoma) and conjugated types (coupled with drugs or toxins), with applications spanning diagnosis (pregnancy detection, HIV, hepatitis testing) and therapy (cancer, autoimmune disorders, infections, asthma, osteoporosis), though they face limitations including laborious production, potential retrovirus contamination, and adverse effects like infusion reactions and immunosuppression.

Monoclonal antibodies are immunoglobulins from single B-cell clones, structurally similar to IgG. Types include chimeric (75% human, 25% mouse), humanized (95% human), and fully human antibodies. Naming conventions indicate origin: '-mab' for monoclonal, '-ximab' for chimeric. They are used in rheumatology, oncology, dermatology, and infectious diseases, enabling precise targeting of disease mechanisms.

Antigen-antibody reactions are characterized by specificity, rapidity, reversibility, and spontaneity, with key properties including affinity (binding strength at a single site), valence (number of binding sites), and avidity (total binding strength). These reactions form the basis for various diagnostic techniques including hemagglutination for blood typing, Coombs test for hemolytic anemia, immunofluorescence for tissue visualization, ELISA for quantitative detection, Western blot for protein identification, and flow cytometry for cell population analysis.

This comprehensive section covers the fundamental principles and techniques of laboratory tests based on antigen-antibody reactions. The biotin-streptavidin interaction forms the strongest non-covalent bond in nature, with one streptavidin molecule binding four biotin molecules, enabling powerful detection systems in ELISA and other assays. Western blot combines electrophoretic separation with antibody detection to identify specific proteins in complex samples. Lateral flow immunassays use pre-deposited antigens on nitrocellulose membranes for rapid visual detection. Immunohistochemistry analyzes subcellular structures in tissue sections using primary antibodies, biotin-conjugated secondary antibodies, and enzyme-substrate systems. Detection methods include biotin-avidin-peroxidase and alkaline phosphatase-anti-alkaline phosphatase systems. Staining patterns (nuclear, cytoplasmic, membrane) indicate protein localization. Immunocytochemistry analyzes subcellular structures in cells in suspension, requiring permeabilization. Fluorescence is a phenomenon where fluorophores absorb light at specific wavelengths and emit light at longer wavelengths. Immunofluorescence microscopy uses fluorophore-conjugated antibodies, with direct and indirect variants. ANA testing uses indirect immunofluorescence on HEp-2 cells, with patterns classified as homogeneous, speckled, peripheral, nucleolar, rim, or cytoplasmic. Testing for mitosis is crucial because cells in different cell cycle phases show different staining patterns. Confocal microscopy provides three-dimensional imaging by collecting images at different focal planes. Isotype control is essential for accounting for non-specific background signals. Flow cytometry displays fluorescence intensity versus cell count as histograms. All antigen-antibody based diagnostic tests share common principles: specific antibody-antibody binding as the fundamental detection mechanism, different detection systems, different solid supports, washing steps, and controls for accurate interpretation.

Antigen-antibody based experiments detect proteins through specific binding interactions, where primary antibodies bind to target antigens and secondary antibodies (labeled with detection molecules like enzymes or fluorophores) enable visualization or quantification; different techniques (ELISA, Western Blot, immunohistochemistry, immunofluorescence, flow cytometry) vary in their detection methods and applications, with ELISA providing quantitative protein concentration measurements, Western Blot showing protein bands for semi-quantitative analysis, immunohistochemistry localizing protein expression in tissues, immunofluorescence using fluorescent labels for cellular localization, and flow cytometry enabling cell sorting based on protein expression.

Diagnostic techniques based on antigen-antibody reactions utilize the high specificity of antibodies to detect and identify specific antigens in samples. These techniques include immunofluorescence (using fluorescent markers for visualization), immunohistochemistry (using enzyme-conjugated antibodies for stable color reactions), ELISA (enzyme-linked immunosorbent assay for liquid samples like serum), and immunochromatography (rapid lateral flow tests like pregnancy tests). The primary antibodies can be polyclonal (from immunized animals, containing multiple antibody clones) or monoclonal (from hybridoma cells, highly specific and consistent). These methods are essential for diagnosing diseases, characterizing tumors, and detecting pathogens in clinical and research settings.

Flow cytometry is a powerful immunological technique for analyzing heterogeneous cell populations. The lecture introduces antibody-based detection methods including Western blotting (protein separation on gels followed by antibody detection), ELISA (antibody binding in microtiter plates), and rapid diagnostic tests like pregnancy and COVID antigen tests (using capillary action on filter paper with antibody-coated regions). These techniques share the fundamental principle of antibody-antigen specificity, where antibodies bind only to their target antigens. The common thread is that all these methods leverage the high specificity of antibody interactions to detect and quantify target molecules in various sample formats, from liquid solutions to solid surfaces.

The immune system protects the body through innate immunity (natural, always present) and acquired immunity (humoral and cell-mediated responses involving T and B cells), but can cause damage through hypersensitivity reactions (immediate, cell-mediated, immune complex-mediated, and antibody-mediated types) and autoimmune diseases where the immune system attacks the body's own antigens, including systemic lupus erythematosus (antibody deposition in tissues), rheumatoid arthritis (joint swelling from extra-vascular tissue inflammation), and Sjögren's syndrome (dry eyes and mouth from salivary and lacrimal gland destruction); immunodeficiency conditions like combined immunodeficiency syndrome (immature T and B lymphocytes), AIDS (HIV attacking the immune system), and HIV infection (single-stranded RNA retrovirus with gp120 proteins causing immune system failure) further demonstrate how immune system dysfunction leads to disease.

Antibody deficiency diseases include agammaglobulinemia (complete absence of immunoglobulins), hypogammaglobulinemia (reduced immunoglobulins), hyper-IgM syndrome (defect in isotype switching), and common variable immunodeficiency (CVID). Patients present with recurrent bacterial infections from extracellular bacteria, autoimmune diseases (thrombocytopenia, thyroiditis, lupus), lymphadenopathy, and increased allergic reactions. CVID is the most common primary immunodeficiency requiring medical attention (1 in 600-700 individuals), characterized by reduced immunoglobulins across all classes and impaired antibody responses. Most patients are asymptomatic, but some have recurrent respiratory infections from encapsulated bacteria. Autoimmune manifestations and allergic reactions may occur. CVID is associated with HLA-B8 and mutations in genes involved in isotype switching (TACI, TNFRSF13B, IL-21).

Allergies are hypersensitive immune responses to allergens (dust mites, pollen, animal dander). Mechanism: IgE antibodies bind mast cells/basophils, releasing histamine, serotonin, heparin causing sneezing, watery eyes, breathing difficulty. Examples: Bronchial asthma (excess bronchial mucus), Hay fever (conjunctival inflammation), Anaphylactic shock (most dangerous, can cause death). Autoimmune diseases occur when immune system attacks self-cells: Rheumatoid arthritis (synovial membrane inflammation), Graves' disease, Pernicious anemia, Insulin-dependent diabetes, Hashimoto's disease, Multiple sclerosis, Scleroderma, Systemic lupus erythematosus. Immunodeficiency disorders: SCID (ADA enzyme deficiency, T/B cell maturation failure), AIDS (HIV infection, T helper cell count drops from 900 to <200 million/L).

Immunopathology studies diseases where immune system abnormalities play a central role. Immune-mediated diseases include allergies (excessive responses to non-infectious antigens), autoimmune diseases (adaptive responses against self-antigens), and autoinflammatory diseases (excessive innate responses). Immunodeficiency specifically refers to fundamental immune compromise causing recurrent infections. Factors increasing infection frequency include antibiotic use (disrupting microbiota), smoking (doubling bacterial infection risk in children), and vitamin D deficiency (meta-analyses show supplementation reduces respiratory infections). Antibody deficiencies cause recurrent bacterial infections from encapsulated bacteria (pneumococcus, Haemophilus). Primary causes include X-linked agammaglobulinemia (complete B cell absence) and Common Variable Immunodeficiency (CVID, ~1 in 20,000 people) with dysfunctional B cells, autoimmunity, and lymphoproliferative manifestations.

Immunopathology studies diseases from immune system dysfunction. Hypersensitivity is exaggerated immune responses causing tissue damage: Type I (immediate IgE-mediated allergic reactions, e.g., hay fever, anaphylaxis); Type II (cytotoxic antibody-mediated cell destruction, e.g., autoimmune hemolytic anemia); Type III (immune complex deposition in tissues, e.g., systemic lupus erythematosus); Type IV (cell-mediated T-cell reactions, e.g., contact dermatitis, transplant rejection). Autoimmunity occurs when immune system attacks body's own tissues (rheumatoid arthritis, multiple sclerosis, type 1 diabetes). Immune deficiency weakens the immune system, increasing susceptibility to infections (congenital or acquired, caused by genetic defects, malnutrition, certain medications, or HIV/AIDS).