Innate and Adaptive Immunity: Microbiology Study Guide

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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 protecting 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, is specific, slower to respond initially, but provides immunological memory.

Comparison Table: Features of Innate vs. Adaptive Immunity

Feature	Innate Immunity	Adaptive Immunity
Bacterial Microbiota	Yes	No
Present at birth	Yes	No
Non-specific immune response	Yes	No
Faster immune response	Yes	No (slower at first, faster upon re-exposure)
Physical barriers	Yes (skin, mucous membranes)	No
Inflammation, fever, complement activation	Yes	No (complement can be activated by antibodies)
Phagocytosis	Yes (macrophages, neutrophils)	No (but can be enhanced by antibodies)
Antibody production	No	Yes (B cells)
Activated T-helper cells	No	Yes
Immunological memory	No	Yes
Specific immune response	No	Yes
Slower immune response	No	Yes (primary response)
Cytokines production	Yes	Yes

Key Concepts in Immunology

Antigens and Epitopes

Antigen: Any substance that can induce an immune response. Epitopes are specific regions on antigens recognized by antibodies or T-cell receptors.

Definition: An epitope is the part of an antigen that is recognized by the immune system.
Example: A protein on the surface of a virus may have multiple epitopes.

Physical Barriers and First Line of Defense

Physical barriers are the body's initial protection against pathogens.

Skin: Acts as a mechanical barrier; secretes antimicrobial peptides.
Mucous membranes: Trap pathogens and contain immune cells.
Other factors: pH, enzymes (lysozyme), and normal microbiota.

Inflammation

Inflammation is a complex response to infection or injury, involving immune cells, blood vessels, and molecular mediators.

Key events: Vasodilation, increased permeability, migration of leukocytes, tissue repair.
Benefits: Isolates pathogens, recruits immune cells, promotes healing.
Process: Margination (leukocytes adhere to vessel walls), diapedesis (movement into tissues).

Phagocytosis

Phagocytosis is the process by which certain cells (e.g., macrophages, neutrophils) engulf and destroy pathogens.

Steps: Chemotaxis, adherence, ingestion, digestion.
Cells involved: Macrophages, neutrophils, dendritic cells.

Complement System

The complement system consists of plasma proteins that enhance immune responses.

Functions: Opsonization, lysis of pathogens, inflammation.
Activation pathways: Classical, alternative, lectin.

Adaptive Immunity: Humoral and Cellular Responses

Adaptive immunity involves highly specific responses mediated by lymphocytes.

B cells: Produce antibodies (humoral immunity).
T cells: Helper T cells (CD4+) activate other immune cells; cytotoxic T cells (CD8+) kill infected cells.
Immunological memory: Faster and stronger response upon re-exposure to the same antigen.

Antibody Structure and Function

Antibodies (immunoglobulins) are proteins produced by B cells that bind to specific antigens.

Structure: Y-shaped molecules with variable and constant regions.
Types: IgG, IgM, IgA, IgE, IgD.
Function: Neutralization, opsonization, complement activation.
Equation for antibody titer:

Vaccines

Vaccines stimulate adaptive immunity by exposing the body to antigens in a controlled manner.

Types:
- Live attenuated (weakened pathogens)
- Inactivated (killed pathogens)
- Subunit (purified antigens)
- Toxoid (inactivated toxins)
- Conjugate (antigen linked to carrier protein)
Example: The polio vaccine is an inactivated vaccine.

Serological Tests

Serological tests detect antibodies or antigens in blood to diagnose infections.

ELISA (Enzyme-Linked Immunosorbent Assay): Detects and quantifies antibodies or antigens.
Direct vs. Indirect tests: Direct tests detect antigens; indirect tests detect antibodies.
Crosslinking: Used to measure antibody concentration (titer).

Hypersensitivity and Autoimmunity

Hypersensitivity refers to exaggerated immune responses; autoimmunity is when the immune system attacks self-tissues.

Types of hypersensitivity: Type I (allergy), Type II (cytotoxic), Type III (immune complex), Type IV (delayed).
Examples: Asthma, blood transfusion reactions, contact dermatitis.

Cells of the Immune System

Various cells play roles in immunity:

Phagocytes: Macrophages, neutrophils
Lymphocytes: B cells, T cells
Other: Dendritic cells, mast cells, natural killer (NK) cells

Key Terms and Definitions

Antigen: Substance that induces an immune response.
Antibody: Protein produced by B cells that binds antigens.
Epitope: Specific part of antigen recognized by immune system.
Phagocytosis: Engulfment and digestion of pathogens by cells.
Complement: Group of proteins that enhance immune responses.
Immunological memory: Ability to respond more rapidly and effectively to previously encountered pathogens.

Additional info:

Adaptive immunity is further classified into active (natural infection or vaccination) and passive (transfer of antibodies, e.g., maternal antibodies).
IgG titer is higher in secondary response compared to primary response due to memory cells.
Loss of self-tolerance can lead to autoimmune diseases.
Serological tests are essential for diagnosis and monitoring of immune responses.