Immunology and Host Defense Mechanisms: Study Notes for Microbiology

Immunology and Host Defense Mechanisms

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. It can be acquired naturally (by infection) or artificially (by vaccination).

• Passive Immunity: Immunity acquired by receiving pre-formed antibodies from another individual. It 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.

Examples:

• Vaccination triggers artificially acquired active immunity.

• Receiving antivenom is an example of artificially acquired passive immunity.

• Maternal antibodies passed to a baby through breast milk provide naturally acquired passive immunity.

Vaccines and Immunization

Vaccines are biological preparations that provide acquired immunity to specific diseases. They can be classified based on their composition and method of production.

• Attenuated (Live) Vaccines: Contain weakened forms of the pathogen. They 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). They are safer but may require booster doses.

• Subunit Vaccines: Contain only specific antigens from the pathogen, reducing the risk of side effects.

• Toxoid Vaccines: Contain inactivated toxins to protect against diseases caused by bacterial toxins.

• Combination Vaccines: Protect against multiple diseases with a single injection.

Key Points:

• Contact immunity can be produced by attenuated vaccines.

• Inactivated vaccines are made from pathogens that cannot replicate.

• Toxoid vaccines provide immunity to the toxin, not the microorganism itself.

Serological Testing and Antibody Assays

Serological tests detect the presence of antibodies or antigens in a patient's blood and are essential for diagnosing infectious diseases and monitoring immune responses.

• ELISA (Enzyme-Linked Immunosorbent Assay): Used to detect and quantify antibodies or antigens. It is sensitive, specific, and can be used for both diagnostic and research purposes.

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

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

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

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

Antibody Structure and Function

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

• Structure: Each antibody consists of two heavy chains and two light chains, with variable regions forming the antigen-binding sites.

• Classes: The most prevalent antibody class in blood is IgG. Other classes include IgM, IgA, IgE, and IgD.

• B Cell Receptors (BCRs): Membrane-bound antibodies on B cells that recognize specific antigens.

Key Points:

• Each B lymphocyte has a unique BCR due to random genetic recombination.

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

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 and mediate humoral immunity.

• T Lymphocytes: Include helper T cells (CD4+), cytotoxic T cells (CD8+), and regulatory T cells. They mediate cellular immunity.

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

• Cluster of Differentiation (CD): Surface proteins used to distinguish different lymphocyte subsets.

Example: Clonal deletion of T lymphocytes occurs in the thymus.

Antigen Presentation and MHC Molecules

Antigen presentation is the process by which cells display antigen fragments on their surface, bound to major histocompatibility complex (MHC) molecules, for recognition by 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.

• Dendritic Cells: Professional antigen-presenting cells that process and present antigens to T cells.

Innate Immunity and First Lines of Defense

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

• Physical Barriers: Skin, mucous membranes.

• Chemical Barriers: Sebum (contains salts and lysozyme), acidic pH of skin, antimicrobial peptides (defensins).

• Cellular Defenses: Phagocytes (macrophages, neutrophils), NK cells, dendritic cells.

• Microbial Antagonism: Normal microbiota compete with pathogens for resources.

Example: Lysozyme in tear fluid is an antibacterial enzyme that breaks down bacterial cell walls.

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, cytokines.

• Edema: Swelling caused by increased vascular permeability during inflammation.

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

Phagocytosis and Pathogen Recognition

Phagocytosis is the process by which phagocytes engulf and destroy pathogens. Recognition of pathogens involves pattern recognition receptors (PRRs) such as TLRs and NOD proteins.

• Toll-like Receptors (TLRs): Recognize microbial surface structures and activate immune responses.

• NOD Proteins: Detect microbial molecules inside the cell.

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

Complement System

The complement system is a group of proteins that enhance immune responses by promoting inflammation, opsonization, and lysis of pathogens.

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

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

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

Hypersensitivity and Autoimmunity

Hypersensitivity refers to exaggerated immune responses to antigens, while autoimmunity is the immune system's attack on self-tissues.

• Autoimmune Responses: Normally prevented by clonal deletion and regulatory T cells.

• Memory Responses: Secondary immune responses are faster and stronger due to memory cells.

Tables

Table 1: Types of Immunity

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

Table 2: Antibody Classes

Class	Main Function	Location
IgG	Main antibody in blood, crosses placenta	Blood, extracellular fluid
IgM	First antibody produced in response	Blood, lymph
IgA	Protects mucosal surfaces	Mucus, saliva, tears
IgE	Involved in allergies, defense against parasites	Bound to mast cells
IgD	Function unclear, B cell receptor	B cell surface

Table 3: Vaccine Types and Features

Vaccine Type	Composition	Advantages	Disadvantages
Attenuated	Live, weakened pathogen	Strong, long-lasting immunity	Risk for immunocompromised
Inactivated	Killed pathogen	Safe, stable	Weaker response, boosters needed
Subunit	Antigenic fragments	Fewer side effects	Weaker response
Toxoid	Inactivated toxin	Protects against toxin effects	May require boosters

Key Definitions

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

• Opsonization: The process by which pathogens are marked for ingestion and destruction by phagocytes.

• Clonal Deletion: The elimination of self-reactive lymphocytes during immune development.

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

• Interferons: Proteins produced in response to viral infection that inhibit viral replication.

Important Equations

• Antibody-Antigen Binding:

$\text{Antibody} + \text{Antigen} \rightarrow \text{Immune Complex}$

• Complement Activation (Classical Pathway):

$\text{C1} \xrightarrow{\text{binds to antibody-antigen complex}} \text{C4} \rightarrow \text{C2} \rightarrow \text{C3} \rightarrow \text{C5} \rightarrow \text{MAC}$

Additional info:

• Some content was inferred and expanded for clarity and completeness, such as the structure of antibody classes and the mechanisms of vaccine action.

• Tables were constructed to summarize and compare key concepts based on standard microbiology knowledge.