Immunology: Innate and Adaptive Immunity, Microbiome, Antibodies, Cellular Responses, Vaccines, and Hypersensitivity

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Immunology Overview

Innate vs. Adaptive Immunity

The immune system is divided into innate and adaptive immunity, each with distinct mechanisms and functions for protecting the body against infection.

Innate Immunity: The body's first line of defense, present from birth, and responds rapidly to pathogens. It includes physical barriers, chemical factors, and cellular components.
Adaptive Immunity: Develops after exposure to antigens, is highly specific, and involves memory. It includes humoral (antibody-mediated) and cellular responses.
Comparison: Innate immunity is non-specific and immediate, while adaptive immunity is specific and slower but provides long-lasting protection.

Innate Defenses

Innate defenses protect the body through various mechanisms:

Physical Barriers: Skin, mucous membranes, and the mucociliary escalator prevent pathogen entry.
Chemical Barriers: Sebum, perspiration, lysozyme, and gastric juice inhibit microbial growth.
Cellular Defenses: Leukocytes (white blood cells) such as neutrophils, macrophages, and natural killer cells recognize and destroy pathogens.
Microbiome: The collection of commensal microorganisms that compete with pathogens and support immune function.

Leukocytes and Immune Response

Leukocytes play key roles in generating immune responses:

Neutrophils: Phagocytose and kill bacteria.
Macrophages: Engulf pathogens and present antigens to T cells.
Lymphocytes: Include B cells (produce antibodies) and T cells (mediate cellular immunity).
Natural Killer Cells: Destroy infected or abnormal cells.

Phagocytosis

Phagocytes kill microbes through a multi-step process:

Chemotaxis: Movement toward infection site.
Adherence: Binding to the pathogen.
Ingestion: Engulfing the pathogen.
Digestion: Lysosomal enzymes destroy the microbe.

Inflammation

Inflammation is a protective response to infection or injury.

Signs: Redness, heat, swelling, pain.
Functions: Isolates pathogens, recruits immune cells, promotes healing.
Steps: Vasodilation, increased permeability, migration of leukocytes, tissue repair.

Complement System

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

Activation: Begins with cleavage of C3, leading to opsonization, inflammation, and lysis of microbes.
Pathways: Classical, alternative, and lectin pathways.

Equation:

Other Innate Factors

Interferons: Proteins that inhibit viral replication.
Antimicrobial Peptides: Directly kill microbes.
Transferrin: Binds iron, limiting microbial growth.

The Microbiome

Germ-Free Mice and Microbiome Functions

Germ-free mice are used to study the microbiome's role in health and immunity.

Functions: Compete with pathogens, aid digestion, synthesize vitamins, modulate immunity.
Colonization: Begins at birth; influenced by birth method, feeding, and contact.
Complementing Barriers: Microbiome supports physical and chemical barriers of innate immunity.
Anatomical Sites Free of Microbes: Blood, cerebrospinal fluid, internal organs.

Adaptive Immunity

Antigens and Antibodies

Adaptive immunity targets specific antigens using antibodies and specialized cells.

Antigen: A molecule recognized by the immune system.
Epitope (Antigenic Determinant): The specific part of an antigen recognized by antibodies.
Antibody Structure: Y-shaped molecule with variable regions binding to antigens.

Diagram: Antibody consists of two heavy and two light chains, with antigen-binding sites at the tips.

Classes of Antibodies

There are five main classes of antibodies, each with unique functions:

Class	Structure	Function	Abundance
IgG	Monomer	Main antibody in blood; crosses placenta	Most abundant
IgM	Pentamer	First produced in response	Second most abundant
IgA	Dimer	Protects mucosal surfaces	Third most abundant
IgE	Monomer	Allergy and parasite defense	Least abundant
IgD	Monomer	B cell receptor	Low abundance

B Cell Activation and Antibody Production

B cells are activated by antigens, undergo clonal expansion, and differentiate into plasma cells that produce antibodies.

Activation: Antigen binds to B cell receptor.
Clonal Expansion: B cell divides to produce identical cells.
Differentiation: Some become plasma cells, others memory B cells.

Equation:

Antibody Functions

Antibodies clear microbes by:

Neutralization: Blocking pathogen attachment.
Agglutination: Clumping pathogens for easier removal.
Opsonization: Enhancing phagocytosis.
Complement Activation: Triggering lysis.
ADCC (Antibody-Dependent Cell-Mediated Cytotoxicity): Recruiting cells to destroy large pathogens.

Cellular Response

Cellular immunity involves T cells and antigen-presenting cells (APCs).

Antigen Presentation: APCs display antigens via MHC I or II molecules.
CD8 T Lymphocyte Activation: APCs activate CD8 T cells, which become cytotoxic T lymphocytes (CTLs).
CTLs: Destroy infected cells using perforin and granzyme.
Natural Killer Cells: Kill cells lacking MHC I.
Regulatory Cells: Shut down immune responses when not needed.

Immunological Memory and Hypersensitivity

Immunological Memory

There are four types of immunological memory, ensuring rapid response to previously encountered antigens.

Primary Response: First exposure, slower.
Secondary Response: Subsequent exposures, faster and stronger.
Memory B Cells: Persist for long-term immunity.
Memory T Cells: Provide cellular memory.

Hypersensitivity

Hypersensitivity is an exaggerated immune response causing tissue damage.

Type I: Immediate (allergy, anaphylaxis).
Type II: Cytotoxic (antibody-mediated).
Type III: Immune complex-mediated.
Type IV: Delayed (cell-mediated).

Vaccines and Herd Immunity

Principles of Vaccination

Vaccines stimulate the immune system to develop protection against diseases.

Herd Immunity: When enough people are immune, disease spread is limited.
How Vaccines Work: Expose the immune system to antigens, generating memory without causing disease.

Types of Vaccines

Type	Description	Booster Needed?
Live	Contains live pathogens	No
Live Attenuated	Weakened pathogens	Sometimes
Killed	Inactivated pathogens	Yes
Subunit	Contains parts of pathogens	Yes
DNA	Contains DNA encoding antigens	Yes

Recombinant Vaccine: Uses genetically engineered antigens.
DNA Vaccine: Uses DNA to produce antigens in the host.

Challenges in Vaccination

Persistence of Diseases: Measles and whooping cough persist due to incomplete vaccination coverage.
Reasons for Not Vaccinating: Safety concerns, misinformation, religious beliefs, access issues.

Key Vocabulary

Term	Definition
Innate immunity	Non-specific, immediate defense mechanisms
Adaptive immunity	Specific, acquired defense with memory
Mucociliary escalator	Mechanism moving mucus and trapped particles out of the respiratory tract
Sebum	Oily secretion from skin, inhibits microbes
Perspiration	Sweat, contains antimicrobial substances
Lysozyme	Enzyme breaking down bacterial cell walls
Gastric juice	Acidic fluid in stomach, destroys pathogens
Plasma	Liquid component of blood
Leukocytes	White blood cells
Platelets	Cell fragments involved in clotting
Leukocytosis	Increase in white blood cells
Leukopenia	Decrease in white blood cells
Phagocytosis	Engulfment and digestion of microbes
Inflammation	Response to injury or infection
Complement system	Proteins enhancing immune responses
Humoral Immunity	Antibody-mediated immunity
Cellular immunity	T cell-mediated immunity
Antigens	Molecules recognized by immune system
Epitope	Specific part of antigen recognized by antibody
Clonal selection	Process of selecting specific lymphocytes
MHC I	Major histocompatibility complex class I
MHC II	Major histocompatibility complex class II
Antigen presentation	Display of antigens by APCs
Clonal expansion	Multiplication of activated lymphocytes
T-dependent antigens	Require T cell help for B cell activation
T-independent antigens	Activate B cells without T cell help
Cytokines	Signaling proteins in immunity
Interleukins	Cytokines between leukocytes
Chemokines	Cytokines attracting cells to infection
Affinity	Strength of antibody-antigen binding
Agglutination	Clumping of particles
Opsonization	Coating pathogens for phagocytosis
ADCC	Antibody-dependent cell-mediated cytotoxicity
Neutralization	Blocking pathogen activity
Granzyme	Enzyme inducing apoptosis
Perforin	Protein forming pores in target cells
Hypersensitivity	Excessive immune response
Anaphylaxis	Severe allergic reaction
Mutualism	Both organisms benefit
Commensalism	One benefits, other unaffected
Parasitism	One benefits, other harmed
Herd immunity	Population-level protection
Adjuvant	Substance enhancing vaccine response
Booster	Additional vaccine dose
Conjugate vaccine	Linked antigens for better response
Toxoid	Inactivated toxin used in vaccines
Attenuated	Weakened pathogen

Additional info: Academic context and definitions were expanded for clarity and completeness. Tables were inferred and structured for comparison and classification.