Immunological Assays and Autoimmune Diseases: Microbiology Study Notes

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Antigen-Antibody Interactions

Overview of Antigen-Antibody Binding

The interaction between antigens and antibodies forms the basis for many immunoassays and diagnostic procedures in microbiology and immunology.

Specificity: Antibodies can detect and bind specifically to their corresponding antigens.
Applications: Used in serological tests to identify microbes via specific antigens.
Outcomes of Interaction:
- Precipitation
- Agglutination
- Activation of complement

Affinity: Strength of binding between antibody and a single epitope or hapten. Avidity: Overall binding strength between antibody and a multivalent antigen.

Example: IgM antibodies are pentameric and have higher avidity than IgG antibodies, which are monomeric.

Formula:

Agglutination Reactions in Immunoassays

Principles and Types of Agglutination

Agglutination involves the clumping of particulate (insoluble) antigens by antibodies. It is widely used in blood typing and pathogen detection.

Direct Agglutination: Antibodies directly cause clumping of antigens (e.g., RBCs).
Indirect (Passive) Agglutination: Soluble antigens are attached to beads or particles; antibodies cause these beads to clump.

Coombs Test

Detects antibodies bound to RBCs (direct) or in serum (indirect).
Used in diagnosis of hemolytic disease of the newborn.

Applications

Blood typing
Detection of pathogens
Pregnancy testing (hCG detection)
COVID-19 lateral flow assays

Enzyme-Linked Immunosorbent Assay (ELISA)

Principles and Types of ELISA

ELISA is a solid-phase immunoassay used for detecting antigens or antibodies in clinical samples. It is highly sensitive and can be automated for large-scale testing.

Direct ELISA: Detects antigens immobilized on a plate using enzyme-conjugated antibodies.
Indirect ELISA: Detects antibodies in serum by binding to immobilized antigens, followed by enzyme-conjugated secondary antibodies.

Chromogenic substrates are used to produce a color change, indicating the presence of the target molecule.

Formula:

Immunofluorescence and Flow Cytometry

Immunofluorescence

Immunofluorescence uses antibodies labeled with fluorescent dyes to detect antigens in tissues or cells.

Direct method: Fluorescent antibody binds directly to antigen.
Indirect method: Unlabeled antibody binds antigen, then fluorescent secondary antibody binds the first antibody.

Flow Cytometry

Flow cytometry measures cell size, granularity, and expression of cell surface or intracellular proteins using fluorescently labeled antibodies.

Cells pass through a laser beam; fluorescence is detected and analyzed.
Can sort cells based on fluorescence intensity.

Western Blot (Immunoblotting)

Principles and Applications

Western blotting separates proteins by gel electrophoresis, transfers them to a membrane, and detects specific proteins using labeled antibodies.

Used to confirm HIV infection after positive ELISA.
Detects specific viral proteins in patient samples.

Monoclonal Antibodies

Production and Applications

Monoclonal antibodies are produced from a single B cell clone and recognize a single epitope. They are generated by fusing B cells with myeloma cells to create hybridomas.

Hybridoma technology: B cells (antibody-producing) fused with myeloma cells (immortalized).
Hybridomas are selected in HAT medium; only fused cells survive.
Monoclonal antibodies are harvested from culture fluid.

Applications

Diagnostics: Pregnancy tests, pathogen detection, quantifying drugs in blood.
Therapeutics: Immunotoxins, treatment of autoimmune diseases, cancer therapy.

Autoimmune Diseases

Diseases Caused Mostly by Antibodies

Autoimmune Hemolytic Anemia: Antibodies target RBC antigens, causing hemolysis.
Myasthenia Gravis: Antibodies block acetylcholine receptors, causing muscle weakness.
Graves' Disease: Antibodies stimulate thyroid hormone production, leading to hyperthyroidism.
Systemic Lupus Erythematosus (SLE): Autoantibodies target nuclear components, causing multi-organ damage.
Hashimoto's Thyroiditis: Antibodies destroy thyroid tissue, causing hypothyroidism.

Diseases Caused Mostly by T Cells

Multiple Sclerosis (MS): T cells attack myelin in the CNS, causing neurological symptoms.
Type 1 Diabetes Mellitus (T1DM): T cells destroy insulin-producing beta cells in the pancreas.
Rheumatoid Arthritis (RA): T cells and antibodies cause joint inflammation and destruction.
Psoriasis: T cell-mediated inflammation of the skin.

Why Autoimmune Diseases Are More Common in Women

X-chromosome factors: Many immune-related genes are on the X chromosome.
Hormonal factors: Estrogen influences immune responses.
Pregnancy: Immune modulation during pregnancy can affect disease expression.

Summary Table: Autoimmune Diseases

Disease	Main Effector	Target	Symptoms
Autoimmune Hemolytic Anemia	Antibody	RBC antigens	Anemia, jaundice
Myasthenia Gravis	Antibody	Acetylcholine receptor	Muscle weakness
Graves' Disease	Antibody	TSH receptor	Hyperthyroidism
Systemic Lupus Erythematosus	Antibody	Nuclear antigens	Multi-organ damage
Hashimoto's Thyroiditis	Antibody/T cell	Thyroid antigens	Hypothyroidism
Multiple Sclerosis	T cell	Myelin	Neurological symptoms
Type 1 Diabetes Mellitus	T cell	Beta cells (pancreas)	Hyperglycemia
Rheumatoid Arthritis	T cell/Antibody	Joint tissue	Pain, swelling, deformity
Psoriasis	T cell	Skin	Scaly patches

Key Terms

Antigen: Any substance that can induce an immune response.
Antibody: Immunoglobulin protein produced by B cells that binds specifically to antigens.
Epitope: The specific part of an antigen recognized by an antibody.
Hybridoma: Cell line produced by fusing a B cell with a myeloma cell, used for monoclonal antibody production.
Immunoassay: Laboratory technique that uses antigen-antibody interactions for detection.

Additional info:

Some explanations and context have been expanded for clarity and completeness.
Table entries and formulas have been inferred and formatted for academic use.