Immunology and Host Defense Mechanisms: Study Notes for Microbiology

Immunological Therapies and Assays

Passive and Active Immunotherapy

Immunotherapy refers to treatments that use components of the immune system to fight diseases, including infections and cancer. There are two main types:

• Passive immunotherapy: Administration of pre-formed antibodies to provide immediate protection or treatment. Example: Antivenom for snake bites.

• Active immunotherapy: Stimulation of the patient's own immune system to produce a response, often through vaccination.

Combination therapies may use both passive and active approaches for enhanced effect.

Antibody Assays

Antibody assays are laboratory tests used to detect the presence of antibodies or antigens in a sample. Common types include:

• ELISA (Enzyme-Linked Immunosorbent Assay): Used to detect and quantify antibodies or antigens. It is sensitive and specific, and can be used for pregnancy tests (detecting hCG hormone).

• Complement fixation test: Detects the presence of specific antibodies by measuring complement activation.

• Direct immunofluorescence test: Uses fluorescent-labeled antibodies to detect antigens in tissues or cells.

• Neutralization assay: Measures the ability of antibodies to neutralize pathogens.

• Immunochromatographic assay: Rapid tests (e.g., pregnancy tests) using membrane filters and labeled antibodies.

Key facts about ELISA:

• Can be used to detect antibody or antigen.

• Are quantitative (can measure concentration).

• Require small amounts of serum.

• May use fluorescent or enzyme-labeled antibodies.

• Often involve membrane filters.

Vaccines and Immunity

Types of Vaccines

Vaccines are biological preparations that provide immunity to specific diseases. Types include:

• Attenuated (live) vaccines: Contain weakened forms of the pathogen. Can produce contact immunity and lifelong protection, but may cause disease in immunocompromised individuals.

• Inactivated (killed) vaccines: Contain killed pathogens or fragments. Safer than live vaccines, but may require boosters.

• Toxoid vaccines: Contain inactivated toxins (e.g., tetanus toxoid).

• Subunit vaccines: Contain only specific antigens or fragments of the pathogen.

• Combination vaccines: Protect against multiple diseases.

• DNA vaccines: Use genetic material to induce immunity (experimental).

Contact immunity is produced by attenuated vaccines, where vaccinated individuals can pass immunity to others.

Immunity Types

• Natural active immunity: Acquired through infection.

• Artificial active immunity: Acquired through vaccination.

• Natural passive immunity: Transfer of antibodies from mother to child (e.g., via placenta or breast milk).

• Artificial passive immunity: Administration of antibodies (e.g., antitoxins).

Antibody Structure and Function

B Cell Receptors (BCRs) and Antibodies

BCRs are membrane-bound immunoglobulins on B lymphocytes that recognize specific antigens.

• Formed in response to antigen encounter.

• Each B cell has unique variable regions, allowing recognition of diverse antigens.

• Each BCR has two antigen-binding sites.

• Estimated diversity: at least $10^{11}$ different BCRs per person.

• Antibody-binding site is made up of variable regions of both heavy and light chains.

Antibody Classes

• IgG: Most prevalent in blood; provides long-term immunity.

• IgM: First antibody produced in response to infection.

• IgA: Found in mucosal areas and secretions.

• IgE: Involved in allergic responses.

• IgD: Functions mainly as a B cell receptor.

Immune System Cells and Functions

Lymphocytes

• B lymphocytes: Produce antibodies; mature in bone marrow.

• T lymphocytes: Regulate immune responses; mature in thymus.

• CD molecules: "Cluster of differentiation" markers used to identify lymphocyte subtypes.

Other Immune Cells

• Macrophages: Phagocytose pathogens and present antigens.

• Dendritic cells: Present antigens to T cells; initiate immune responses.

• Neutrophils: First responders to infection; phagocytic.

• Eosinophils: Attack parasitic helminths; release toxins.

• Basophils: Release histamine; involved in inflammation.

• NK cells: Kill virus-infected and tumor cells.

Antigen Presentation and Recognition

MHC Molecules

• Class I MHC: Present endogenous antigens (from inside the cell) to CD8+ T cells.

• Class II MHC: Present exogenous antigens (from outside the cell) to CD4+ T cells.

Antigen Processing

• Dendritic cells and macrophages process and present antigens to T cells.

• Clonal deletion in thymus prevents autoimmunity by eliminating self-reactive T cells.

Innate Immunity and Host Defenses

First Line of Defense

• Physical barriers: Skin, mucous membranes.

• Chemical barriers: Sebum (contains salts and acids), lysozyme (in tears), acidic pH of skin.

• Microbial antagonism: Normal microbiota compete with pathogens.

Second Line of Defense

• Phagocytic cells: Macrophages, neutrophils, dendritic cells.

• Antimicrobial chemicals: Interferons, defensins, complement proteins.

• Inflammation: Increases blood flow, recruits immune cells.

• Fever: Increases effectiveness of interferons, inhibits pathogen growth.

Phagocyte Recognition of Pathogens

• TLRs (Toll-like receptors): Bind surface structures of microbes.

• NOD proteins: Detect microbial molecules inside cells.

• Lectins: Bind carbohydrates on microbial surfaces.

• Opsonins: Molecules that enhance phagocytosis.

Complement System

Activation Pathways

• Classical pathway: Triggered by antibody-antigen complexes.

• Alternative pathway: Begins with C3b binding to microbial surfaces.

• Lectin pathway: Initiated by lectin binding to carbohydrates on microbes.

Functions

• Opsonization (enhancing phagocytosis)

• Inflammation (C5a, C3a)

• Cell lysis (formation of MAC - membrane attack complex)

Inflammation and Chemotaxis

Inflammatory Mediators

• Histamine: Increases vascular permeability, contributes to edema.

• Leukotrienes: Promote inflammation.

• Prostaglandins: Mediate fever and pain.

Chemotaxis

Chemotaxis is the movement of a cell toward or away from a chemical stimulus, crucial for immune cell recruitment to infection sites.

Host Resistance and Species Barriers

• Protection from infection can result from absence of receptors or unsuitable environment for microbial attachment.

• Salty, acidic skin and presence of phagocytes contribute to resistance.

Summary Table: Types of Immunity

Type of Immunity	How Acquired	Example
Natural Active	Infection	Recovery from measles
Artificial Active	Vaccination	MMR vaccine
Natural Passive	Maternal antibodies	Antibodies in breast milk
Artificial Passive	Injection of antibodies	Antivenom

Key Equations and Concepts

• Antibody diversity: $10^{11}$ different BCRs per person

• Complement activation (alternative pathway): $C3b$ binding to microbe surface

Additional info:

• Some context and definitions were inferred to clarify fragmented notes and questions.

• Examples and explanations were expanded for clarity and completeness.