Innate and Adaptive Immunity, Host-Pathogen Interactions, and Immunological Applications

Innate and Adaptive Immunity

Overview of the Immune System

The immune system protects the body from pathogens through a complex network of cells, tissues, and molecules. It is divided into two main branches: innate immunity and adaptive immunity.

• Innate Immunity: The first line of defense, present from birth, non-specific, and responds rapidly to pathogens.

• Adaptive Immunity: Develops after exposure to antigens, is highly specific, and has memory for faster responses upon re-exposure.

Comparison Table: Innate vs. Adaptive Immunity

Feature	Innate Immunity	Adaptive Immunity
Specificity	Non-specific	Highly specific
Memory	None	Present
Response Time	Immediate (minutes to hours)	Delayed (days)
Main Components	Physical barriers, phagocytes, NK cells, complement	B cells, T cells, antibodies

Normal Microbiota and the Immune System

• Normal microbiota are the microorganisms that reside on and within the human body without causing disease.

• They compete with pathogens for nutrients and space, produce antimicrobial substances, and stimulate immune responses.

• Disruption of normal microbiota (e.g., by antibiotics) can increase susceptibility to infections.

First Line of Defense: Barriers

• Mechanical Barriers: Physical processes that remove microbes (e.g., skin shedding, mucociliary escalator, flushing by tears and urine).

• Chemical Barriers: Substances that inhibit or destroy pathogens (e.g., lysozyme in tears, acidic pH of stomach, fatty acids in skin oils).

• Physical Barriers: Structures that block pathogen entry (e.g., intact skin, mucous membranes).

Antimicrobial Peptides

• Short proteins produced by host cells that disrupt microbial membranes, inhibit cell wall synthesis, or interfere with metabolism.

• Examples: Defensins, cathelicidins.

Lymphoid Tissues

• Primary lymphoid tissues: Sites of lymphocyte development (bone marrow and thymus).

• Secondary lymphoid tissues: Sites where immune responses are initiated (lymph nodes, spleen, tonsils, MALT).

Leukocytes (White Blood Cells)

• Neutrophils: Phagocytose bacteria and fungi; most abundant.

• Lymphocytes: B cells (produce antibodies), T cells (cell-mediated immunity), NK cells (kill infected cells).

• Monocytes/Macrophages: Phagocytosis and antigen presentation.

• Eosinophils: Combat parasites and participate in allergic responses.

• Basophils/Mast cells: Release histamine; involved in inflammation and allergies.

Cytokines

• Small proteins that mediate and regulate immunity, inflammation, and hematopoiesis.

• Types include:

  • Interleukins (IL): Communication between leukocytes.

  • Interferons (IFN): Antiviral responses.

  • Tumor Necrosis Factors (TNF): Inflammation and apoptosis.

  • Chemokines: Chemotaxis of immune cells.

Interferons

• Produced in response to viral infections.

• Induce antiviral states in neighboring cells, activate NK cells and macrophages.

Iron-Binding Proteins

• Limit microbial growth by sequestering free iron (essential for many pathogens).

• Examples: Transferrin, lactoferrin, ferritin.

Complement System

• A group of plasma proteins that enhance ("complement") immune responses.

• Activated via three pathways: classical, alternative, lectin.

• Outcomes: Opsonization, inflammation, cell lysis (via membrane attack complex).

Inflammation

• Protective response to infection or injury.

• Four cardinal signs: redness (rubor), heat (calor), swelling (tumor), pain (dolor).

• Caused by increased blood flow, vascular permeability, and leukocyte migration.

Fever

• Systemic increase in body temperature, usually due to pyrogens (e.g., IL-1, TNF-alpha).

• Enhances immune function and inhibits pathogen growth.

Adaptive Immunity

Overview and Diagram of Adaptive Immunity

Adaptive immunity involves highly specific responses to antigens and generates immunological memory. It is mediated by lymphocytes (B and T cells).

• Humoral immunity: Mediated by B cells and antibodies.

• Cell-mediated immunity: Mediated by T cells.

Antigens and Immunogenicity

• Antigen: Any substance that can be recognized by the immune system and elicit an immune response.

• Immunogenicity: The ability of an antigen to provoke an immune response.

T Cell Types

• Cytotoxic T cells (TC, CD8+): Kill infected or abnormal cells.

• Helper T cells (TH, CD4+): Activate B cells, other T cells, and macrophages.

• Regulatory T cells (TReg): Suppress immune responses to maintain tolerance.

Self-Tolerance Screening

• Process by which developing lymphocytes are tested for self-reactivity.

• Failure leads to autoimmunity.

T Cells vs. B Cells

• T cells: Mature in thymus, involved in cell-mediated immunity, recognize antigens presented by MHC molecules.

• B cells: Mature in bone marrow, produce antibodies, recognize free antigens.

Major Histocompatibility Complex (MHC)

• MHC I: Present on all nucleated cells; present endogenous antigens to CD8+ T cells.

• MHC II: Present on antigen-presenting cells; present exogenous antigens to CD4+ T cells.

Antigen Presentation

• Intracellular Antigen Presentation: Endogenous antigens are processed and presented on MHC I to cytotoxic T cells.

• Extracellular Antigen Presentation: Exogenous antigens are processed and presented on MHC II to helper T cells.

T Cell Activation, Proliferation, and Differentiation

• Requires antigen presentation, co-stimulatory signals, and cytokines.

• Activated T cells proliferate and differentiate into effector and memory cells.

Cytotoxic T Cell Action

• Recognize infected cells via MHC I-antigen complexes and induce apoptosis using perforin and granzymes.

B Cell Activation

• T-dependent antigens: Require help from T helper cells for activation.

• T-independent antigens: Can activate B cells without T cell help (usually polysaccharides).

• Activated B cells proliferate and differentiate into plasma cells (antibody production) and memory B cells.

Antibody Structure and Functions

• Y-shaped molecules composed of two heavy and two light chains.

• Functions: Neutralization, opsonization, complement activation, agglutination, antibody-dependent cellular cytotoxicity (ADCC).

Antibody Types

• IgG: Most abundant, crosses placenta, long-term immunity.

• IgM: First produced, pentamer, effective in agglutination.

• IgA: Found in mucosal areas and secretions.

• IgE: Involved in allergy and defense against parasites.

• IgD: Functions mainly as a B cell receptor.

Immunological Memory

• Memory B and T cells enable a faster and stronger response upon re-exposure to the same antigen.

Categories of Humoral Immunity

• Natural active: Infection-induced immunity.

• Natural passive: Maternal antibodies.

• Artificial active: Vaccination.

• Artificial passive: Antibody therapy.

Immunodeficiencies and Autoimmunity

• Primary immunodeficiencies: Genetic defects present at birth.

• Secondary immunodeficiencies: Acquired due to infection, malnutrition, or therapy.

• Autoimmunity: Immune response against self-antigens.

Biomedical Applications: Vaccines, Diagnostics, and Therapeutics

Smallpox Eradication

• Achieved through global vaccination campaigns, surveillance, and containment.

• Possible due to lack of animal reservoir, effective vaccine, and visible symptoms.

Jenner’s Vaccination and the Term "Vaccine"

• Edward Jenner used cowpox to protect against smallpox, coining the term "vaccine" from vacca (Latin for cow).

Herd Immunity

• Occurs when a high proportion of the population is immune, reducing disease spread and protecting non-immune individuals.

Vaccine Categories

Type	Example	Benefits	Risks
Live attenuated	MMR, Varicella	Strong, long-lasting immunity	Risk in immunocompromised
Inactivated	Polio (IPV)	Safe, stable	Weaker immunity, boosters needed
Subunit/conjugate	Hepatitis B, Hib	Few side effects	May require adjuvants
Toxoid	Tetanus, Diphtheria	Targets toxins	Boosters needed
mRNA	COVID-19	Rapid development	New technology, rare side effects

Agglutination Reactions and Blood Typing

• Agglutination: Clumping of particles due to antibody-antigen binding.

• Used in blood typing to detect ABO and Rh antigens.

ELISA (Enzyme-Linked Immunosorbent Assay)

• Used to detect antigens or antibodies in samples.

• Types:

  • Direct ELISA: Detects antigen with labeled antibody.

  • Indirect ELISA: Detects antibody using antigen-coated wells and labeled secondary antibody.

  • Sandwich ELISA: Uses capture and detection antibodies to "sandwich" the antigen.

Western Blotting

• Detects specific proteins using gel electrophoresis, transfer to membrane, and antibody probing.

Host-Microbe Interactions and Pathogenesis

Tropism

• The specificity of a pathogen for a particular host tissue, determined by host receptors and pathogen factors.

Virulence Factors

• Molecules that enable pathogens to colonize, evade immunity, and cause disease (e.g., toxins, adhesins, invasins).

ID50 and LD50

• ID50: Infectious dose for 50% of the population.

• LD50: Lethal dose for 50% of the population.

Basic Reproduction Number (R0)

• Average number of secondary cases generated by one case in a susceptible population.

• High R0 indicates high transmissibility.

Endotoxins and Exotoxins

• Endotoxin: Lipopolysaccharide (LPS) from Gram-negative bacteria; causes inflammation and septic shock.

• Exotoxins: Proteins secreted by bacteria; highly potent and specific (e.g., botulinum toxin).

Steps to Infection

1. Entry

2. Adhesion

3. Invasion

4. Multiplication

5. Exit

Portals of Entry and Exit

• Entry: Skin, mucous membranes, respiratory tract, GI tract, urogenital tract.

• Exit: Same as entry or via secretions, excretions, or blood.

Adhesion Factors

• Molecules (e.g., fimbriae, pili, adhesins) that allow pathogens to attach to host cells.

Biofilm and Quorum Sensing

• Biofilm: Community of microbes attached to a surface, protected by extracellular matrix.

• Quorum sensing: Cell-to-cell communication to coordinate gene expression based on population density.

Invasins

• Enzymes or proteins that help pathogens invade host tissues (e.g., hyaluronidase, collagenase).

Immune Evasion Mechanisms

• Antigenic variation, capsule formation, inhibition of phagocytosis, intracellular survival.

Transmission Modes and Precautions

Mode	Description	Precautions
Contact	Direct or indirect physical contact	Hand hygiene, gloves
Droplet	Large respiratory droplets (cough/sneeze)	Masks, distance
Airborne	Small particles suspended in air	N95 respirators, negative pressure rooms

Additional info: Where diagrams were referenced (e.g., Figs 12.2, 12.9, 12.10, 12.16), descriptions have been provided in text form. For tables (e.g., Table 10.5), main content has been inferred based on standard microbiology knowledge.