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 acquired naturally (infection) or artificially (vaccination).

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

Examples:

• Vaccination triggers artificially acquired active immunity.

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

• Maternal antibodies passed to a baby is naturally 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 but may not be suitable for immunocompromised individuals.

• Inactivated (Killed) Vaccines: Contain pathogens that have been killed or inactivated. Safer than live vaccines but may require booster doses.

• Toxoid Vaccines: Contain inactivated toxins produced by bacteria. Used for diseases where toxins cause illness (e.g., tetanus).

• Subunit Vaccines: Contain only specific antigens from the pathogen, not the whole organism.

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

Contact immunity can be produced by attenuated vaccines, as the weakened pathogen can sometimes spread to others, providing indirect protection.

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 the diagnosis of infectious diseases and immune status.

• ELISA (Enzyme-Linked Immunosorbent Assay): Used to detect and quantify antibodies or antigens. Can be direct or indirect. Involves enzyme-labeled antibodies and a color change reaction.

• Immunochromatographic Assay: Rapid tests (e.g., pregnancy tests) that use antibodies bound to a membrane to detect specific antigens.

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

• Direct Immunofluorescence Test: Uses fluorescently labeled antibodies to detect antigens in a sample.

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

Key Points about ELISA:

• Can detect antibody or antigen.

• Not quantitative unless specifically designed as such.

• Require small amounts of serum.

• Do not require large amounts of serum or membrane filters.

Antibody Structure and Function

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

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

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

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

Table: Antibody Classes and Functions

Class	Main Function
IgG	Most abundant; crosses placenta; opsonization, neutralization
IgM	First produced in response; pentamer; agglutination
IgA	Secretions (mucus, saliva); mucosal immunity
IgE	Allergic responses; defense against parasites
IgD	B cell receptor (function not fully understood)

Lymphocytes and Clonal Selection

Lymphocytes are white blood cells essential for adaptive immunity. They include B cells and T cells, each with unique receptors generated by gene rearrangement.

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

• T Lymphocytes: Mediate cellular immunity; mature in thymus.

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

• Cluster of Differentiation (CD): Surface proteins used to identify lymphocyte subtypes (e.g., CD4, CD8).

Antigen Presentation and MHC

Antigen-presenting cells (APCs) process and present antigens to T cells using major histocompatibility complex (MHC) molecules.

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

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

Example: Dendritic cells phagocytose microbes, process antigens, and display them on MHC II molecules to activate helper T cells.

Innate Immunity and First/Second Lines of Defense

The innate immune system provides immediate, non-specific defense against pathogens. It includes physical barriers, chemical factors, and cellular defenses.

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

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

Table: Innate Immune Defenses

Defense	Function
Skin	Physical barrier; acidic pH; sebum inhibits microbes
Lysozyme	Enzyme in tears/saliva; breaks down bacterial cell walls
Phagocytes	Engulf and destroy pathogens
Complement	Protein cascade; opsonization, lysis, inflammation
Interferons	Antiviral proteins; inhibit viral replication

Phagocyte Recognition and Pathogen Detection

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

• TLRs: Bind to conserved microbial structures on the phagocyte membrane.

• NOD Proteins: Detect microbial molecules inside the cell.

• Lectins: Bind to carbohydrates on microbial surfaces.

Complement System

The complement system is a group of plasma proteins that enhance immune responses. Activation can occur via classical, alternative, or lectin pathways.

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

• MAC (Membrane Attack Complex): Forms pores in pathogen membranes, leading to lysis.

Example: A mutation in C3 protein can impair phagocyte attraction and MAC formation, reducing immune effectiveness.

Inflammation and Chemotaxis

Inflammation is a protective response to infection or injury, characterized by redness, heat, swelling, and pain. Chemotaxis is the movement of cells toward chemical signals at infection sites.

• Histamine and Leukotrienes: Mediate vasodilation and increased vascular permeability, contributing to edema.

• Chemotaxis: Attracts immune cells to sites of infection.

Fever and Antimicrobial Substances

Fever is an increase in body temperature that can enhance immune responses and inhibit pathogen growth.

• Benefits of Fever: Increases effectiveness of interferons, inhibits viral replication, enhances immune cell activity.

• Antimicrobial Substances: Lysozyme (in tears), defensins, interferons, complement proteins.

Cells of the Immune System

Various white blood cells play specialized roles in immunity.

• Macrophages: Phagocytosis and antigen presentation.

• Dendritic Cells: Antigen presentation to T cells.

• Neutrophils: Rapid responders; phagocytosis.

• Eosinophils: Defense against parasites; release toxins onto helminths.

• Basophils: Release histamine in allergic responses.

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

Autoimmunity and Clonal Deletion

Autoimmunity is prevented by eliminating self-reactive lymphocytes during development (clonal deletion) and by regulatory T cells suppressing autoreactive responses.

• Clonal Deletion: Occurs in thymus (T cells) and bone marrow (B cells).

• Regulatory T Cells: Suppress immune responses to self-antigens.

Summary Table: Types of Immunity

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

Additional info:

• Some content was inferred and expanded for clarity and completeness, such as the structure and function of antibodies, the role of different immune cells, and the mechanisms of vaccine action.

• Tables were constructed to summarize key comparisons and classifications relevant to the questions.