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 pre-formed antibodies from another source. Can be natural (maternal antibodies) or artificial (antibody therapy).

• Natural Immunity: Acquired through natural exposure to pathogens or transfer of antibodies (e.g., breastfeeding).

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

Example: Vaccination triggers an immune response which produces artificial active immunity. A person who contracts a disease and recovers develops naturally acquired active immunity.

Vaccines

Vaccines are preparations that stimulate the immune system to develop protection against specific pathogens. They come in several types:

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

• Inactivated (Killed) Vaccines: Contain killed pathogens. Safer but may require booster doses.

• Toxoid Vaccines: Contain inactivated toxins produced by bacteria. Induce immunity to the toxin rather than the organism.

• Subunit Vaccines: Contain only parts of the pathogen (e.g., proteins, polysaccharides).

• Combination Vaccines: Protect against multiple diseases in one shot.

Contact immunity within a population can be produced by attenuated vaccines.

Example: The oral polio vaccine (OPV) is an attenuated vaccine that can provide contact immunity.

Serological Testing and Antibody Assays

Serological tests detect the presence of antibodies or antigens in a patient's blood, aiding in diagnosis and monitoring of diseases.

• ELISA (Enzyme-Linked Immunosorbent Assay): Used to detect and quantify antibodies or antigens. Can be direct or indirect. Not quantitative, but sensitive and specific.

• Immunochromatographic Assay: Used in rapid tests (e.g., pregnancy tests) to detect specific antigens or antibodies.

• Complement Fixation Test: Detects the presence of specific antibodies by measuring complement consumption.

• Direct Immunofluorescence Test: Uses fluorescent-labeled antibodies to detect antigens in samples.

• 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.

Antibody Structure and Function

Antibodies (immunoglobulins) are Y-shaped proteins produced by B lymphocytes that bind to specific antigens.

• Structure: Consist of two heavy chains and two light chains. The antigen-binding site is formed by the variable regions of both heavy and light chains.

• Classes: IgG, IgM, IgA, IgE, IgD. IgG is the most prevalent antibody class in blood.

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

Example: Each B lymphocyte is randomly generated with antibody variable regions that determine its BCR specificity. Scientists estimate that each person forms at least $10^{11}$ different types of B lymphocytes with distinct BCRs.

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: Involved in cell-mediated immunity; mature in the thymus. Types include helper T cells (CD4+), cytotoxic T cells (CD8+), and regulatory T cells.

• NK Cells: Recognize and kill virus-infected and tumor cells.

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

Example: Clonal deletion of developing T lymphocytes takes place in the thymus.

Antigen Presentation and MHC

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

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

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

Example: Dendritic cells display microbial epitope-MHC II complexes on their surface after phagocytosing and processing a microbe.

Innate Immunity and First/Second Line Defenses

Innate immunity provides immediate, non-specific defense against pathogens. It includes physical, chemical, and cellular barriers.

• First Line of Defense: Skin, mucous membranes, normal microbiota, and secretions (e.g., lysozyme in tears).

• Second Line of Defense: Phagocytic cells (macrophages, neutrophils), inflammation, fever, antimicrobial proteins (complement, interferons).

Example: Lysozyme is an antibacterial enzyme present in tear fluid. Sebum contains salts and acids, creating an inhospitable environment for microbes.

Phagocytosis and Pathogen Recognition

Phagocytes recognize and ingest pathogens using pattern recognition receptors (PRRs) such as Toll-like receptors (TLRs) and NOD proteins.

• TLRs: Bind to surface structures of microbes, triggering immune responses.

• NOD Proteins: Detect microbial molecules inside the cell.

• Opsonins: Molecules that enhance phagocytosis by marking pathogens for ingestion.

Example: TLRs in the phagocyte cytoplasmic membrane bind surface structures of microbes.

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).

Example: The alternative pathway of complement activation begins with C3b binding to the surface of a microbe.

Inflammation and Fever

Inflammation is a protective response to infection or injury, characterized by redness, heat, swelling, and pain. Fever is an increase in body temperature that can enhance immune function.

• Key Mediators: Histamine, leukotrienes, prostaglandins.

• Edema: Swelling due to increased vascular permeability, often mediated by histamine and leukotrienes.

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

Example: Both leukotrienes and histamine contribute to the edema associated with inflammation. Fever increases the effectiveness of interferons.

Cells of the Immune System

Various cells play roles in innate and adaptive immunity:

• Macrophages: Phagocytose pathogens and present antigens.

• Dendritic Cells: Antigen-presenting cells that activate T cells.

• Eosinophils: Attack parasitic helminths and secrete toxins onto virally infected cells.

• Basophils: Release histamine during allergic responses.

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

Example: Eosinophils attach to the surface of parasitic helminths and release toxins that kill the parasite.

Immunological Memory and Secondary Responses

Immunological memory allows the immune system to respond more rapidly and effectively to pathogens that have been encountered previously.

• Primary Immune Response: The initial response to a new antigen.

• Secondary (Memory) Response: Faster and stronger response upon re-exposure to the same antigen.

Example: Vaccination relies on the formation of memory cells to provide long-term protection.

Autoimmunity and Clonal Deletion

Autoimmunity occurs when the immune system attacks the body's own tissues. Clonal deletion in the thymus and bone marrow helps prevent autoimmunity by eliminating self-reactive lymphocytes.

• Regulatory T Cells: Suppress autoimmune responses.

• Clonal Deletion: Removal of self-reactive T and B cells during development.

Example: Clonal deletion of T cells, lack of necessary cytokine signals, and regulatory T cell suppression prevent activation of autoreactive T cells.

Tables

Comparison of Vaccine Types

Vaccine Type	Contents	Advantages	Disadvantages
Attenuated	Weakened live pathogen	Strong, long-lasting immunity; contact immunity	Not for immunocompromised; risk of reversion
Inactivated	Killed pathogen	Safe; stable	Weaker immunity; may need boosters
Toxoid	Inactivated toxin	Protects against toxin-mediated diseases	Requires boosters
Subunit	Pathogen components	Safe; fewer side effects	Weaker immunity; may need adjuvants

Antibody Classes and Functions

Class	Main Location	Function
IgG	Blood, extracellular fluid	Main antibody in secondary response; crosses placenta
IgM	Blood	First antibody produced; effective in agglutination
IgA	Mucosal areas, secretions	Protects mucous membranes
IgE	Bound to mast cells, basophils	Involved in allergic responses, defense against parasites
IgD	B cell surface	Functions as B cell receptor

Additional info: Some explanations and context have been expanded for clarity and completeness based on standard microbiology curriculum.