Immunity, Vaccines, and Host Defenses: Microbiology Study Guide

Immunity and Immunization

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, classified by how they are acquired and their mechanisms.

• Active Immunity: Immunity produced by the body's own immune response after exposure to an antigen. Can be natural (infection) or artificial (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.

• Artificial Immunity: Acquired through medical intervention (e.g., vaccines, antibody injections).

Examples:

• Vaccination = Artificially acquired active immunity

• Maternal antibodies = Naturally acquired passive immunity

• Antibody therapy = Artificially acquired passive immunity

Vaccines

Vaccines are preparations that stimulate an immune response to provide protection against specific diseases. They can be classified by 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 cause disease in 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.

• Toxoid Vaccines: Contain inactivated toxins produced by the pathogen.

• Combination Vaccines: Contain antigens from multiple pathogens.

Key Points:

• Contact immunity is produced by live attenuated vaccines.

• Inactivated vaccines are made from deactivated whole microorganisms or antigenic fragments, but are not made from mutated forms of the pathogen.

• Toxoid vaccines stimulate antibody responses to toxins, not the microorganism itself.

Serological Testing and Antibody Assays

Types of 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. Can be used for pregnancy tests (detecting hCG hormone).

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

• Immunochromatographic Assay: Rapid tests (e.g., pregnancy tests) that use membrane filters and labeled antibodies.

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

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

ELISA Key Facts:

• 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 fluorescent molecules (unless using a fluorescent ELISA variant).

• May use membrane filters in rapid test formats.

Host Defenses: Innate and Adaptive Immunity

First Line of Defense

The first line of defense consists of physical and chemical barriers that prevent pathogen entry.

• Skin: Acts as a physical barrier; sebum creates an acidic environment.

• Mucous Membranes: Trap and remove microbes.

• Normal Microbiota: Compete with pathogens (microbial antagonism).

• Antimicrobial Substances: Lysozyme in tears and saliva breaks down bacterial cell walls.

Second Line of Defense

The second line of defense includes non-specific cellular and chemical responses to pathogens that breach the first line.

• Phagocytic Cells: Macrophages, neutrophils, dendritic cells ingest and destroy microbes.

• Natural Killer (NK) Cells: Recognize and kill virus-infected and abnormal cells.

• Inflammation: Increases blood flow and recruits immune cells to sites of infection.

• Fever: Increases effectiveness of interferons and inhibits microbial growth.

• Antimicrobial Proteins: Interferons (alpha and beta) are effective against viruses; complement proteins enhance phagocytosis and lyse pathogens.

Key Substances:

• Histamine and Leukotrienes: Contribute to edema and inflammation.

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

• Defensins: Antimicrobial peptides produced by neutrophils and epithelial cells.

Phagocyte Recognition of Pathogens

• TLRs (Toll-like Receptors): On phagocyte membranes, bind to microbial surface structures.

• NOD Proteins: Detect microbial molecules inside the cell.

• Lectins: Bind to carbohydrates on microbial surfaces.

• Opsonins: Molecules that enhance phagocytosis by marking pathogens.

Complement System

The complement system is a group of proteins that enhance immune responses.

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

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

• Alternative Pathway: Begins with factor B binding to microbial surfaces.

Adaptive Immunity: Lymphocytes and Antibodies

B Cells and Antibodies

B lymphocytes (B cells) are responsible for humoral immunity by producing antibodies.

• B Cell Receptors (BCRs): Membrane-bound antibodies specific to antigens.

• Antibody Structure: Each antibody has two heavy chains and two light chains; the variable regions of both chains form the antigen-binding site.

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

T Cells

T lymphocytes (T cells) are involved in cell-mediated immunity.

• CD Molecules: Surface proteins used to distinguish T cell subtypes (e.g., CD4, CD8).

• Clonal Deletion: Occurs in the thymus to eliminate self-reactive T cells and prevent autoimmunity.

• Class I MHC: Present endogenous antigens to cytotoxic T cells.

• Class II MHC: Present exogenous antigens to helper T cells.

Autoimmunity and Memory

• Autoimmunity Prevention: Clonal deletion and regulatory T cell suppression prevent self-reactive lymphocytes from causing disease.

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

Tables

Types of Immunity

Type	How Acquired	Example
Natural Active	Infection/exposure to pathogen	Recovering from measles
Artificial Active	Vaccination	MMR vaccine
Natural Passive	Maternal antibodies	IgG crossing placenta
Artificial Passive	Injection of antibodies	Antivenom

Antibody Classes

Class	Main Function	Location
IgG	Long-term immunity, opsonization	Blood, extracellular fluid
IgM	First antibody produced, agglutination	Blood, lymph
IgA	Mucosal immunity	Secretions (tears, saliva, mucus)
IgE	Allergic responses, defense against parasites	Bound to mast cells, basophils
IgD	B cell receptor	B cell surface

Additional info:

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

• Tables were constructed to summarize immunity types and antibody classes, as implied by the questions.