Innate and Adaptive Immunity: Features, Mechanisms, and Clinical Applications

Innate and Adaptive Immunity

Overview of Immune Responses

The immune system is divided into two main branches: innate immunity and adaptive (acquired) immunity. Each branch has distinct features, mechanisms, and roles in defending the body against pathogens.

• Innate Immunity: The first line of defense, present at birth, non-specific, and rapid in response.

• Adaptive Immunity: Develops after exposure to antigens, highly specific, slower to respond initially but provides immunological memory.

Comparison of Innate and Adaptive Immunity Features

Feature	Innate Immunity	Adaptive Immunity
Bacterial Microbiota	✔	✖
Present at birth	✔	✖
Non-specific immune response	✔	✖
Faster immune response	✔	✖
Physical barriers	✔	✖
Inflammation, fever, complement activation	✔	✖
Phagocytosis	✔	✖
Antibody production	✖	✔
Activated T-helper cells	✖	✔
Immunological memory	✖	✔
Specific immune response	✖	✔
Slower immune response	✖	✔
Cytokines production	✔	✔

Antigens and Epitopes

Definition and Properties

Antigens are molecules capable of inducing an immune response. Epitopes are specific regions on antigens recognized by antibodies or T-cell receptors.

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

• Epitope: The specific part of an antigen that is recognized and bound by an antibody or T-cell receptor.

• Example: The influenza virus hemagglutinin protein contains multiple epitopes recognized by the immune system.

Physical Barriers and First Line of Defense

Components and Functions

Physical barriers are the body's initial defense against pathogens, preventing their entry and colonization.

• Skin: Acts as a mechanical barrier; its dryness and acidity inhibit microbial growth.

• Mucous membranes: Trap pathogens and contain antimicrobial substances.

• Other factors: Tears, saliva, and stomach acid contribute to pathogen elimination.

Inflammation

Process and Benefits

Inflammation is a complex biological response to harmful stimuli, such as pathogens or injury. It involves multiple steps and serves to eliminate the cause of cell injury, clear out necrotic cells, and initiate tissue repair.

• Steps of Inflammation:

  1. Vasodilation

  2. Increased vascular permeability

  3. Migration of leukocytes (margination, diapedesis)

  4. Phagocytosis of pathogens

  5. Tissue repair

• Benefits: Localizes infection, removes pathogens, and promotes healing.

Phagocytosis

Mechanism and Cell Types

Phagocytosis is the process by which certain immune cells engulf and destroy pathogens.

• Phagocytic cells: Neutrophils, macrophages, and dendritic cells.

• Steps: Recognition, attachment, engulfment, digestion, and exocytosis of debris.

• Example: Macrophages ingest bacteria in infected tissues.

Complement System

Activation and Effects

The complement system is a group of plasma proteins that enhance immune responses, including opsonization, cell lysis, and inflammation.

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

• Effects: Lysis of pathogens, recruitment of inflammatory cells, and opsonization.

• Example: Complement activation leads to membrane attack complex formation and bacterial cell lysis.

Adaptive Immunity

Cellular and Humoral Responses

Adaptive immunity involves highly specific responses mediated by lymphocytes and the production of antibodies.

• B cells: Mature in bone marrow; responsible for antibody production (humoral immunity).

• T cells: Mature in thymus; responsible for cell-mediated immunity.

• Immunological memory: Memory cells allow for faster and stronger responses upon re-exposure to the same antigen.

• Antibody structure: Pentameric IgM, monomeric IgG, etc.

Equation:

$\text{Rate of antibody production} = k[\text{B cells}][\text{Antigen}]$

Types of Immunity

Classification and Examples

Immunity can be classified as active or passive, and as natural or artificial.

• Active immunity: Body produces its own antibodies (e.g., after infection or vaccination).

• Passive immunity: Antibodies are transferred from another source (e.g., maternal antibodies, antiserum).

• Natural immunity: Acquired through natural exposure.

• Artificial immunity: Acquired through medical intervention (e.g., vaccines).

• Example: Acquired, passive, and natural immunity: maternal IgG crossing the placenta.

Vaccines

Types and Mechanisms

Vaccines stimulate the immune system to develop protection against specific pathogens.

• Live attenuated vaccines: Contain weakened pathogens.

• Inactivated vaccines: Contain killed pathogens.

• Subunit vaccines: Contain purified components of pathogens.

• Toxoid vaccines: Contain inactivated toxins.

• Conjugate vaccines: Combine antigens with carrier proteins to enhance immunogenicity.

Serological Tests

Principles and Applications

Serological tests detect antibodies or antigens in patient samples to diagnose infections and immune status.

• ELISA (Enzyme-Linked Immunosorbent Assay): Detects and quantifies antibodies or antigens.

• Direct tests: Detect antigens directly.

• Indirect tests: Detect antibodies by using known antigens.

• Titer: The concentration of antibodies in serum, often determined by serial dilution.

• Example: High IgG titer in secondary response compared to primary response.

Equation:

$\text{Titer} = \frac{1}{\text{Highest dilution at which antibody is detectable}}$

Hypersensitivity and Autoimmunity

Types and Clinical Manifestations

Hypersensitivity refers to exaggerated immune responses that cause tissue damage. Autoimmunity occurs when the immune system attacks self-antigens.

• Type I: Immediate (allergy, anaphylaxis)

• Type II: Cytotoxic (hemolytic anemia, transfusion reactions)

• Type III: Immune complex (serum sickness)

• Type IV: Delayed (contact dermatitis, transplant rejection)

• Example: Systemic anaphylaxis caused by mast cell degranulation.

Extras: Key Differences Between Innate and Adaptive Immunity

Summary Table

Feature	Innate Immunity	Adaptive Immunity
Speed of response	Immediate (minutes to hours)	Delayed (days)
Specificity	Non-specific	Highly specific
Memory	None	Present
Main cells	Phagocytes, NK cells	B and T lymphocytes
Barriers	Physical, chemical	None

Additional info: Some content inferred and expanded for clarity and completeness, including definitions, examples, and equations.