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 source. 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 artificial active immunity.

• Receiving antivenom provides artificial passive immunity.

• Recovering from chickenpox provides naturally acquired active immunity.

Vaccines

Vaccines are preparations that stimulate the immune system to develop protection against specific pathogens. 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 and can induce contact immunity, but may not be suitable for 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 bacteria.

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

Key Points:

• Contact immunity is produced by attenuated vaccines.

• Inactivated vaccines are made from pathogens that cannot replicate.

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

Serology and Immunoassays

Serology involves the detection of antibodies or antigens in blood serum. Immunoassays are laboratory techniques used to detect and quantify these molecules.

• ELISA (Enzyme-Linked Immunosorbent Assay): Used to detect antibodies or antigens. Can be quantitative or qualitative. The antibody label is an enzyme, not a fluorescent molecule.

• Immunochromatographic Assay: Used in home pregnancy tests to detect hCG hormone.

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

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

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

Table: Comparison of Immunoassays

Assay Type	Detects	Label Used	Quantitative?
ELISA	Antibody/Antigen	Enzyme	Yes/No
Immunofluorescence	Antigen	Fluorescent dye	No
Immunochromatographic	Antigen	Colloidal gold/Enzyme	No
Complement Fixation	Antibody	Complement proteins	No

Antibodies and B Cell Receptors (BCRs)

Antibodies are proteins produced by B lymphocytes that bind specifically to antigens. B cell receptors (BCRs) are membrane-bound antibodies on B cells.

• Each B cell has unique BCRs generated by random recombination of gene segments.

• BCRs are complementary in shape to specific antigenic determinants (epitopes).

• The antibody-binding site is made up of the variable regions of both light and heavy chains.

• There are at least $10^{11}$ different types of B lymphocytes with distinct BCRs.

Antibody Classes:

• 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.

• IgD: Functions mainly as a BCR.

Lymphocytes and Immune Cell Differentiation

Lymphocytes are white blood cells essential for adaptive immunity. They include B cells, T cells, and natural killer (NK) cells.

• CD Molecules: Surface proteins used to identify and differentiate lymphocyte subtypes (e.g., CD4, CD8).

• Clonal Deletion: Process in the thymus (for T cells) or bone marrow (for B cells) that eliminates self-reactive lymphocytes to prevent autoimmunity.

• Once mature, lymphocytes migrate to secondary lymphoid organs (e.g., lymph nodes, spleen).

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 inside the cell) to CD8+ cytotoxic T cells.

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

• Red blood cells (RBCs) do not produce MHC and cannot present antigens.

Innate Immunity and First/Second Lines of Defense

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).

• Microbial Antagonism: Normal microbiota prevent colonization by pathogens.

Table: Components of Innate Immunity

Component	Function
Skin	Physical barrier; acidic pH inhibits microbes
Lysozyme	Enzyme in tears/saliva that breaks down bacterial cell walls
Phagocytes	Engulf and destroy pathogens
Complement	Protein cascade that enhances phagocytosis and lyses microbes
Interferons	Proteins that inhibit viral replication

Phagocyte Recognition and Pathogen Destruction

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 surface of microbes.

• 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. Activation can occur via the classical, alternative, or lectin pathways.

• Alternative Pathway: Begins with factor B binding to the surface of a microbe.

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

• C3b: Key opsonin in the complement system.

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.

• Histamine and Leukotrienes: Contribute to vasodilation and increased vascular permeability, leading to edema.

• Chemotaxis: Movement of immune cells toward chemical signals at the site of infection.

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

Cells of the Immune System

Various cells play specialized roles in immune defense.

• Macrophages: Engulf and destroy pathogens; present antigens to T cells.

• Dendritic Cells: Professional antigen-presenting cells; activate T cells.

• Neutrophils: First responders; phagocytose bacteria.

• 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.

Autoimmunity and Clonal Deletion

Autoimmunity is prevented by eliminating self-reactive lymphocytes through clonal deletion in the thymus (T cells) and bone marrow (B cells).

• Regulatory T Cells: Suppress autoimmune responses.

• Clonal Deletion: Removal of lymphocytes that recognize self-antigens.

Serological Testing and Blood Analysis

Serological tests detect antibodies or antigens in blood for diagnosis of disease.

• Serology: Study of serum and immune responses in body fluids.

• Serologic Tests: Used to detect specific antigens or antibodies.

• Blood Analysis: Can measure the proportion of plasma, concentration of serum proteins, and presence of antigen-specific antibodies.

Summary Table: Types of Immunity

Type	How Acquired	Example
Natural Active	Infection	Recovery from measles
Natural Passive	Maternal antibodies	IgG crossing placenta
Artificial Active	Vaccination	MMR vaccine
Artificial Passive	Injection of antibodies	Antivenom

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