Innate Immunity

Overview of Innate Immunity

Innate immunity is the body's first line of defense against pathogens. It consists of physical, chemical, and cellular defenses that are present from birth and respond rapidly to infections. Unlike adaptive immunity, innate immunity does not provide long-lasting or specific protection.

• Definition: Innate immunity refers to non-specific defense mechanisms that come into play immediately or within hours of an antigen's appearance in the body.

• Key Features: Rapid response, no memory, non-specific recognition of pathogens.

• Examples: Skin, mucous membranes, phagocytic cells, inflammation, fever.

Three Lines of Defense

• First Line: Physical and chemical barriers (e.g., skin, mucous membranes, secretions).

• Second Line: Cellular defenses (e.g., phagocytes), inflammation, fever, antimicrobial substances.

• Third Line: Adaptive immunity (not part of innate immunity, but important for context).

Physical and Chemical Aspects of Skin and Mucous Membranes

• Skin: Acts as a physical barrier; secretes sebum and sweat with antimicrobial properties.

• Mucous Membranes: Trap pathogens; cilia move microbes out of the body.

• Chemical Defenses: Lysozyme in tears and saliva, acidic pH of skin and stomach.

Body's Nonspecific Defenses

• Phagocytosis: Engulfment and destruction of pathogens by phagocytes (e.g., neutrophils, macrophages).

• Inflammation: Localized response to injury or infection, characterized by redness, heat, swelling, and pain.

• Fever: Elevated body temperature that enhances immune function and inhibits pathogen growth.

• Antimicrobial Proteins: Interferons, complement system.

Phagocytosis

• Definition: The process by which phagocytic cells ingest and destroy microbes and debris.

• Steps:

  1. Chemotaxis and adherence

  2. Ingestion

  3. Formation of phagosome

  4. Fusion with lysosome (phagolysosome)

  5. Digestion and destruction

  6. Exocytosis of residual material

• Cells Involved: Neutrophils, macrophages, dendritic cells.

Inflammation

• Purpose: To localize infection, recruit immune cells, and promote tissue repair.

• Signs: Redness, heat, swelling, pain, and sometimes loss of function.

• Process: Release of histamine and other mediators increases blood flow and vascular permeability.

Fever

• Definition: An increase in body temperature above the normal range, often in response to infection.

• Benefits: Inhibits growth of some pathogens, enhances immune responses.

Antimicrobial Substances

• Interferons: Proteins produced by virus-infected cells that inhibit viral replication in nearby cells.

• Complement System: A group of proteins that enhance phagocytosis, lyse pathogens, and promote inflammation.

• Defensins: Small peptides that disrupt microbial membranes.

Normal Microbiota and Innate Immunity

• Role: Compete with pathogens for nutrients and space, produce substances harmful to pathogens, stimulate the immune system.

• Example: Lactobacillus in the vagina produces lactic acid, lowering pH and inhibiting pathogens.

Cells and Organs of the Immune System

• Leukocytes (White Blood Cells): Include neutrophils, eosinophils, basophils, monocytes, and lymphocytes.

• Primary Lymphoid Organs: Bone marrow and thymus (sites of immune cell development).

• Secondary Lymphoid Organs: Lymph nodes, spleen, tonsils (sites of immune response initiation).

Mechanisms of Killing by Phagocytes

• Oxygen-dependent Killing: Production of reactive oxygen species (ROS) to destroy pathogens.

• Oxygen-independent Killing: Use of enzymes and antimicrobial peptides.

Complement System

• Definition: A group of serum proteins that enhance ("complement") the ability of antibodies and phagocytic cells to clear microbes.

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

• Main Functions: Opsonization, chemotaxis, cell lysis, inflammation.

Interferons

• Types: Type I (alpha, beta) and Type II (gamma).

• Function: Inhibit viral replication, activate immune cells.

Inflammatory Mediators

• Histamine: Increases vascular permeability.

• Prostaglandins and Leukotrienes: Promote inflammation and attract immune cells.

Pathogen Evasion of Innate Immunity

• Capsules: Prevent phagocytosis.

• Enzyme Production: Destroy host defenses (e.g., leukocidins, proteases).

• Antigenic Variation: Change surface proteins to avoid recognition.

Summary Table: Innate vs. Adaptive Immunity

Medical Applications

• Immunodeficiencies: Disorders where components of innate immunity are defective, leading to increased susceptibility to infections.

• Therapeutic Use of Interferons: Used in treatment of viral infections and some cancers.