Immunology and Host Defense Mechanisms: Study Notes for Microbiology

Immunological Assays and Antibody Detection

Types of Immunoassays

Immunoassays are laboratory techniques used to detect or quantify specific antigens or antibodies in a sample. They are essential tools in diagnostics and research.

• ELISA (Enzyme-Linked Immunosorbent Assay): Used to detect antibodies or antigens; can be quantitative or qualitative. Utilizes enzyme-linked antibodies and colorimetric detection.

• Immunochromatographic Assay: Rapid tests (e.g., pregnancy tests) that use antibodies bound to a membrane to detect the presence of a target molecule.

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

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

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

Key Points about ELISA:

• Can be used to detect either antibody or antigen.

• May require large amounts of serum.

• Antibody label is often a fluorescent or enzyme molecule.

• Involves the use of membrane filters.

Vaccines and Immunization

Types of Vaccines

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

Type	Description	Example
Attenuated	Live, weakened pathogens	MMR (measles, mumps, rubella)
Inactivated	Killed pathogens or fragments	Polio (Salk), influenza
Toxoid	Inactivated toxins	Tetanus, diphtheria
Subunit	Specific antigenic fragments	Hepatitis B
Combination	Multiple antigens/pathogens	DTaP

Contact Immunity: Can be produced by attenuated vaccines, where vaccinated individuals may spread the weakened pathogen to others, indirectly immunizing them.

Inactivated Vaccines:

• Produced from deactivated whole microorganisms or antigenic fragments.

• Made from pathogens that cannot replicate.

• Generally safer than attenuated vaccines.

• Do not provide contact immunity.

Immunity Types

Immunity can be acquired naturally or artificially, and can be active or passive.

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	Antibody therapy	Antivenom

Antibody Structure and Function

B Cell Receptors (BCRs) and Antibody Diversity

B cell receptors are membrane-bound immunoglobulins on B lymphocytes that recognize specific antigens.

• Formed in response to antigen encounter.

• Complementary in shape to specific antigenic determinants.

• Each B lymphocyte has two antigen-binding sites.

• Antibody diversity is generated by random recombination of variable regions.

• Estimated at least $10^{11}$ different BCRs per person.

Antibody Structure:

• Composed of two heavy chains and two light chains.

• Antigen-binding site consists of variable regions of both heavy and light chains.

Antibody Classes

There are five major classes of antibodies (immunoglobulins): IgG, IgM, IgA, IgD, and IgE.

• IgG: Most prevalent in blood; provides long-term immunity.

• IgM: First antibody produced in response to infection.

• IgA: Found in mucosal areas and secretions.

• IgE: Involved in allergic responses and defense against parasites.

• IgD: Functions mainly as a B cell receptor.

Host Defense Mechanisms

Lines of Defense

The human body has multiple lines of defense against pathogens:

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

• Second Line: Innate immune cells (phagocytes, NK cells, eosinophils), antimicrobial chemicals (defensins, interferons), inflammation, fever.

• Third Line: Adaptive immunity (lymphocytes, antibodies).

Physical and Chemical Barriers

• Skin: Salty, acidic environment; physical barrier.

• Sebum: Contains salts and fatty acids, creating inhospitable conditions for microbes.

• Lysozyme: Enzyme in tears and saliva that breaks down bacterial cell walls.

• Mucous Membranes: Trap pathogens and contain antimicrobial substances.

Innate Immune Cells and Functions

• Phagocytes (macrophages, neutrophils): Engulf and destroy microbes.

• Dendritic Cells: Present antigens to T cells.

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

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

• Basophils: Release histamine during allergic responses.

Inflammation and Fever

• Inflammation: Characterized by redness, heat, swelling, and pain. Mediated by histamine, leukotrienes, and prostaglandins.

• Edema: Swelling due to increased vascular permeability; histamine and leukotrienes are key mediators.

• Fever: Higher temperature increases effectiveness of interferons and inhibits viral replication.

Complement System

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

• Activated via classical, alternative, or lectin pathways.

• Alternative pathway begins with factor B binding to the surface of a microbe.

• Complement activation leads to opsonization, inflammation, and formation of the membrane attack complex (MAC).

Recognition of Pathogens

Pattern Recognition Receptors

• TLRs (Toll-like Receptors): Bind surface structures of microbes on phagocyte membranes.

• NOD Proteins: Detect microbial components inside cells.

• Lectins: Bind carbohydrates on microbial surfaces.

Antigen Presentation

• Class I MHC: Presents endogenous antigens (from within the cell).

• Class II MHC: Presents exogenous antigens (from outside the cell).

Lymphocyte Development and Function

B and T Lymphocytes

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

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

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

• Cluster of Differentiation (CD): Surface proteins used to identify lymphocyte subtypes.

Memory and Primary Immune Responses

• Primary Immune Response: First exposure to antigen; slower and less robust.

• Memory Response: Subsequent exposures; faster and stronger due to memory cells.

Pathogen Evasion and Host Limitations

Red Blood Cells and Immune Evasion

• RBCs do not produce MHC, so they cannot present antigens.

• RBCs do not enter lymphoid tissue.

• Complement cannot effectively destroy RBCs infected by intracellular parasites (e.g., malaria).

• RBCs do not produce cytokines necessary for adaptive immunity.

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 by which pathogens are marked for phagocytosis.

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

• Autoimmunity: Immune response against self-antigens.

• Microbial Antagonism: Competition between normal microbiota and pathogens.

Summary Table: Host Defenses

Line of Defense	Main Components	Function
First	Skin, mucous membranes, secretions	Prevent entry of pathogens
Second	Phagocytes, inflammation, fever, complement	Non-specific destruction of invaders
Third	B and T lymphocytes, antibodies	Specific, adaptive immunity

Additional info:

• Some content inferred from context and standard microbiology curriculum, such as the details of vaccine types and immune cell functions.

• Tables reconstructed to summarize and compare key concepts.