Immunology and Host Defenses: Microbiology Study Guide

Immunology and Host Defenses

Types of Immunity

Immunity refers to the body's ability to resist or eliminate potentially harmful foreign materials or abnormal cells. There are several types of immunity, each with distinct characteristics and mechanisms.

• Active Immunity: Immunity produced by the body in response to exposure to an antigen. Can be naturally acquired (infection) or artificially acquired (vaccination).

• Passive Immunity: Immunity acquired by receiving preformed antibodies from another source. Can be natural (maternal antibodies) or artificial (antibody therapy).

• Natural Immunity: Acquired through natural exposure to pathogens.

• Artificial Immunity: Acquired through medical intervention, such as vaccines or antibody injections.

Example: Vaccination triggers artificially acquired active immunity, while receiving antivenom is artificially acquired passive immunity.

Vaccines

Vaccines are preparations that stimulate an immune response to provide protection against specific diseases. They can be classified based on their composition and method of production.

• Attenuated (Live) Vaccines: Contain weakened forms of the pathogen. Provide strong, long-lasting immunity and can induce contact immunity, but may not be suitable for immunocompromised individuals.

• Inactivated (Killed) Vaccines: Contain killed pathogens or inactivated toxins (toxoids). Safer than live vaccines but may require boosters.

• Subunit Vaccines: Contain only specific antigens from the pathogen.

• Combination Vaccines: Protect against multiple diseases in a single shot.

• DNA Vaccines: Contain genetic material encoding antigens.

Example: The polio vaccine is available in both live attenuated (oral) and inactivated (injectable) forms.

Comparison of Vaccine Types

Vaccine Type	Main Features	Example
Attenuated	Live, weakened pathogen; strong, long-lasting immunity; risk for immunocompromised	MMR, oral polio
Inactivated	Killed pathogen; safer, may need boosters	Injected polio, influenza
Toxoid	Inactivated toxins; targets toxin, not organism	Tetanus, diphtheria
Subunit	Specific antigens; fewer side effects	Hepatitis B
Combination	Multiple diseases in one	DTaP

Serological Testing and Immunoassays

Serological tests detect the presence of antibodies or antigens in blood and are essential for diagnosing infections and immune status.

• ELISA (Enzyme-Linked Immunosorbent Assay): Used to detect and quantify antibodies or antigens. Highly sensitive and specific.

• Immunochromatographic Assay: Rapid tests (e.g., pregnancy tests) that use antibodies to detect specific molecules.

• Complement Fixation Test: Measures the ability of antibodies to activate complement.

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

• Neutralization Assay: Measures the ability of antibodies to neutralize pathogens or toxins.

Example: A home pregnancy test is an immunochromatographic assay that detects hCG hormone in urine.

ELISA: Key Features

• Can be used to detect either antibody or antigen.

• Highly sensitive and specific.

• Quantitative or qualitative results.

• Uses enzyme-labeled antibodies and colorimetric detection.

Antibody Structure and Function

Antibodies (immunoglobulins) are Y-shaped proteins produced by B cells that specifically bind antigens.

• Structure: Composed of two heavy chains and two light chains, with variable regions forming the antigen-binding site.

• Classes: IgG, IgM, IgA, IgE, IgD (with IgG being the most prevalent in blood).

• B Cell Receptors (BCRs): Membrane-bound antibodies on B cells, each with unique specificity.

Example: The variable regions of both heavy and light chains form the antigen-binding site.

Lymphocytes and Immune Response

Lymphocytes are white blood cells essential for adaptive immunity. They include B cells, T cells, and natural killer (NK) cells.

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

• T Lymphocytes: Mediate cellular immunity; mature in thymus. Subtypes include helper T cells (CD4+), cytotoxic T cells (CD8+), and regulatory T cells.

• NK Cells: Part of innate immunity; kill virus-infected and tumor cells.

Clonal Deletion: Process by which self-reactive lymphocytes are eliminated during development, primarily in the thymus (T cells) and bone marrow (B cells).

Antigen Presentation and MHC

Major Histocompatibility Complex (MHC) molecules present antigens to T cells.

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

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

Example: Red blood cells do not express MHC and cannot present antigens.

Innate Immunity: First and Second Lines of Defense

Innate immunity provides immediate, non-specific defense against pathogens.

• First Line: Physical and chemical barriers (skin, mucous membranes, sebum, lysozyme, acidic pH).

• Second Line: Cellular defenses (phagocytes, NK cells), inflammation, fever, antimicrobial proteins (complement, interferons).

Example: Lysozyme in tears and saliva breaks down bacterial cell walls.

Table: First vs. Second Line of Defense

First Line	Second Line
Skin, mucous membranes, secretions (sebum, lysozyme)	Phagocytes, inflammation, fever, complement, interferons

Phagocytosis and Pathogen Recognition

Phagocytes (e.g., macrophages, neutrophils) ingest and destroy pathogens. Recognition involves pattern recognition receptors (PRRs) such as TLRs and NOD proteins.

• TLRs (Toll-like Receptors): Recognize microbial surface structures.

• NOD Proteins: Detect microbial molecules inside cells.

• Opsonins: Molecules that enhance phagocytosis by marking pathogens.

Example: TLRs in phagocyte membranes bind to microbial components, triggering immune responses.

Complement System

The complement system is a group of proteins that enhance immune responses, including opsonization, inflammation, and cell lysis.

• Activation Pathways: Classical, alternative, and lectin pathways.

• Key Components: C3b (opsonization), C5a (chemotaxis), MAC (membrane attack complex).

Equation:

$\text{C3 convertase} + \text{C3} \rightarrow \text{C3a} + \text{C3b}$

Inflammation and Fever

Inflammation is a localized response to infection or injury, characterized by redness, heat, swelling, and pain. Fever is a systemic increase in body temperature that enhances immune function.

• Key Mediators: Histamine, leukotrienes, prostaglandins.

• Edema: Swelling due to increased vascular permeability.

• Fever: Increases effectiveness of interferons and inhibits pathogen replication.

Example: Histamine and leukotrienes contribute to edema during inflammation.

Cells of the Immune System

• Phagocytes: Macrophages, neutrophils, dendritic cells.

• Eosinophils: Attack parasitic helminths and modulate allergic responses.

• Basophils: Release histamine during allergic reactions.

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

Example: Eosinophils attach to helminths and release toxins to kill the parasite.

Immunological Memory and Secondary Responses

Immunological memory allows for a faster and stronger response upon re-exposure to the same antigen.

• Primary Response: First exposure; slower, lower magnitude.

• Secondary (Memory) Response: Subsequent exposures; rapid and robust due to memory cells.

Example: Vaccinated individuals mount a rapid response upon exposure to the pathogen.

Autoimmunity and Immune Regulation

Autoimmunity occurs when the immune system attacks self-tissues. Regulatory T cells and clonal deletion help prevent autoimmunity.

• Clonal Deletion: Removal of self-reactive lymphocytes during development.

• Regulatory T Cells: Suppress autoreactive immune responses.

Key Terms and Definitions

• Antigen: A molecule capable of inducing an immune response.

• Antibody: A protein produced by B cells that binds specifically to an antigen.

• Opsonization: The process of marking pathogens for phagocytosis.

• Chemotaxis: Movement of cells toward or away from a chemical stimulus.

• Epitope: The specific part of an antigen recognized by an antibody.

• Serology: The study of serum and immune responses in body fluids.

Additional info: Some content and explanations have been expanded for clarity and completeness based on standard microbiology and immunology curricula.