Innate Immune System Overview and Mechanisms

Overview of Immune System

• Immune System Overview

  • Comprises innate and adaptive immunity.

  • Innate immunity is nonspecific, present at birth, and always on.

Innate Immunity Characteristics

• Definition: Immunity that is present from birth, functioning all the time without prior exposure to pathogens.

• Nonspecific Defense: Not tailored to particular pathogens.

• First Line of Defense:

  • Physical Barriers:

  • Skin and Mucous Membranes: As the body's first line of defense against pathogens (e.g. bacteria, viruses).

    • Skin: Composed of dermis and epidermis, protects against pathogen entry.

    • Mucous Membranes: Found in GI, respiratory, and urinary tracts; they protect internal surfaces.

• Other Physical Factors:

  • Cilia: Help expel mucus and pathogens from respiratory tract.

  • Urine and Vaginal Secretions: Flush out microbes from the urethra/vagina.

  • Defecation, Vomiting, and Diarrhea: Mechanisms the body uses to expel harmful microbes.

Chemical Factors in Innate Immunity

• Definition: Chemical components that contribute to immunity.

• Examples:

  • Sebum: Protective oil on skin that prevents bacterial growth.

  • Earwax: Contains fatty acids that inhibit the growth of bacteria and fungi.

  • Normal Microbiota: Naturally occurring bacteria that prevent the colonization of harmful pathogens.

Second Line of Defense

• Activated when the first line fails. Still part of innate immunity but more specific.

• White Blood Cells (WBCs):

  • Key role in fighting infections.

  • Types of WBCs:

  • Neutrophils: First responders to infection, most abundant (60-70%).

  • Lymphocytes: (involved more in adaptive immunity)

  • Monocytes: Can differentiate into macrophages, act in later stages of infection.

  • Eosinophils and Basophils: Involved in responses to parasites and allergies.

Phagocytosis Process

• Definition: The process by which phagocytes (like neutrophils and macrophages) engulf and digest pathogens.

• Steps:

  1. Chemotaxis: Chemical signaling attracts phagocytes to the site of infection.

  2. Adherence: Binding of phagocyte to pathogen via receptors (PAMPs and TLRs).

  3. Ingestion: Engulfing the pathogen to form a phagosome.

  4. Digestion: Fusion of phagosome with lysosome to form phagolysosome, where the pathogen is broken down.

Inflammation Process

• Purpose: To destroy and eliminate injurious agents and repair tissues.

• Signs of Inflammation: Redness, heat, swelling, pain.

• Chemical Mediators:

  • Histamine, Kinins, Prostaglandins, Leukotrienes, Cytokines: Cause blood vessel dilation and increased permeability for easier WBC access.

• Stages:

  • Vasodilation: Increases blood flow to the affected area, allowing more WBCs to reach the site of infection.

  • Margination: WBCs stick to the blood vessel walls in response to cytokines.

Importance in Clinical Context

• Understanding these processes is vital for diagnosing infections and understanding lab values from blood tests. Increased WBC counts can indicate infections, while differentials help identify the type of infection.