Innate Immunity: Structure, Components, and Mechanisms

Innate Immunity

Overview of Innate Immunity

Innate immunity is the body's first line of defense against pathogens, providing rapid and non-specific protection through physical, chemical, and cellular barriers. It is present from birth and does not require prior exposure to pathogens.

• Definition: Resistance to pathogens conferred by barriers, chemicals, cells, and processes that remain unchanged upon subsequent infections with the same pathogens.

• Key Features:

  • Immediate response

  • Broad activity against a wide variety of pathogens (bacteria, viruses, fungi, protozoa, worms)

  • Includes physical and chemical barriers, phagocytic cells, and antimicrobial chemicals

• Example: Skin and mucous membranes act as barriers to prevent pathogen entry.

Body's 1st Line of Defense

Physical and Chemical Barriers

The first line of defense consists of physical and chemical barriers that prevent pathogens from entering the body.

• Skin: The outer layer of skin (epidermis) is composed of tightly packed cells, providing a physical barrier. Epidermal cells are continually shed, removing attached microbes.

• Mucous Membranes: Line the respiratory, digestive, urinary, and reproductive tracts. Mucus traps pathogens and contains antimicrobial substances.

• Other Barriers:

  • Lacrimal apparatus: Produces tears that contain lysozyme, an enzyme that destroys bacteria.

  • Microbiome: Normal microbiota compete with pathogens and produce antimicrobial substances.

  • Antimicrobial peptides/secretions: Present in skin, mucous membranes, and neutrophils; disrupt pathogen membranes.

• Example: Stomach acid destroys ingested pathogens.

Body's 2nd Line of Defense

Cellular and Chemical Components

The second line of defense operates when pathogens penetrate the skin or mucous membranes, involving cells, chemicals, and processes that attack and eliminate invaders.

• Cells:

  • Phagocytes: Macrophages, neutrophils, and dendritic cells engulf and destroy pathogens.

  • Natural Killer (NK) Cells: Destroy virus-infected and cancerous cells by releasing toxins.

• Antimicrobial Chemicals:

  • Toll-like receptors (TLRs) and NOD proteins: Detect pathogen-associated molecular patterns (PAMPs) and trigger immune responses.

  • Interferons and complement proteins: Enhance immune response and pathogen destruction.

  • Lysozyme and antimicrobial peptides: Disrupt pathogen membranes.

• Processes:

  • Phagocytosis: Engulfing and digesting pathogens.

  • Inflammation: Increases blood flow and immune cell migration to infection sites.

  • Fever: Enhances immune response and inhibits pathogen growth.

• Example: Neutrophils release NETs (neutrophil extracellular traps) to capture and kill bacteria.

Major Leukocyte Types and Functions

Hematopoiesis and Leukocyte Classification

Blood stem cells in bone marrow produce erythrocytes, platelets, and leukocytes. Leukocytes are divided into granulocytes and agranulocytes, each with specific roles in immunity.

• Granulocytes:

  • Basophils: Release inflammatory chemicals; not phagocytic.

  • Neutrophils: Phagocytic; release toxic chemicals and NETs.

  • Eosinophils: Phagocytic; combat parasitic worms and allergies.

• Agranulocytes:

  • Monocytes: Leave blood and mature into macrophages; phagocytic cells that devour foreign objects.

  • Lymphocytes: Key cells in adaptive immunity (not covered in detail here).

• Platelets: Involved in blood clotting and inflammation.

• Example: Monocytes become macrophages that engulf bacteria in tissues.

Complement System

Structure and Function

The complement system is a cascade of plasma proteins that enhances the ability of antibodies and phagocytic cells to clear pathogens.

• Key Functions:

  • Opsonization: Coating pathogens to enhance phagocytosis.

  • Cytolysis: Formation of membrane attack complex (MAC) to lyse pathogens.

  • Inflammation: Attracts immune cells to infection sites.

• Pathways:

  • Classical pathway: Triggered by antibodies bound to antigens.

  • Alternative pathway: Triggered by pathogen surfaces.

  • Lectin pathway: Triggered by mannose-binding lectin binding to pathogen surfaces.

• Example: C3 protein is central to all complement pathways.

Pathway	Trigger	Key Proteins
Classical	Antibody-antigen complex	C1, C2, C4
Alternative	Pathogen surface	C3, Factor B, D, P
Lectin	Mannose on pathogen	MBL, MASP-1, MASP-2

Phagocytosis

Stages and Mechanisms

Phagocytosis is the process by which phagocytic cells ingest and destroy pathogens. It involves several distinct stages.

• Chemotaxis: Movement of phagocytes toward infection site in response to chemical signals.

• Adhesion: Phagocyte attaches to pathogen using complementary chemicals.

• Ingestion: Phagocyte engulfs pathogen, forming a phagosome.

• Maturation: Phagosome fuses with lysosome to form phagolysosome.

• Killing: Enzymes and chemicals in phagolysosome destroy pathogen.

• Elimination: Residual debris is expelled from the phagocyte.

• Example: Macrophages ingest and destroy bacteria in tissues.

Pattern Recognition and Response

PAMPs, PRRs, TLRs, and NOD Proteins

Pattern recognition receptors (PRRs) detect pathogen-associated molecular patterns (PAMPs) to initiate immune responses.

• PAMPs: Molecules shared by microbes but absent in humans (e.g., LPS, flagellin).

• PRRs: Receptors that recognize PAMPs and trigger immune responses.

• Toll-like receptors (TLRs): Located on cell surfaces and inside phagocytes; recognize PAMPs and activate immune signaling.

• NOD proteins: Found in cytoplasm; recognize bacterial cell wall components and viral RNA.

• Example: TLR activation leads to cytokine production and upregulation of costimulatory molecules (e.g., CD80/CD86).

PRR Type	Location	Recognized PAMPs
TLRs	Cell surface, endosomes	LPS, flagellin, dsRNA
NOD proteins	Cytoplasm	Bacterial peptidoglycan, viral RNA

Non-Phagocytic Killing

Neutrophil NETs and NK Cells

Some immune cells kill pathogens without phagocytosis, using specialized mechanisms.

• Neutrophil NETs: Neutrophils release extracellular traps composed of DNA and antimicrobial proteins to ensnare and kill pathogens.

• Natural Killer (NK) Cells: Destroy virus-infected and cancerous cells by releasing cytotoxic granules.

• Example: NETs trap both Gram-positive and Gram-negative bacteria, killing them with antimicrobial peptides.

Inflammation and Fever

Role in Innate Immunity

Inflammation and fever are key processes in innate immunity that help eliminate pathogens and promote healing.

• Inflammation: Increases blood flow, recruits immune cells, and promotes tissue repair.

• Fever: Raises body temperature to inhibit pathogen growth and enhance immune function.

• Example: Inflammatory response to bacterial infection includes redness, heat, swelling, and pain.

Summary Table: Key Cells in Innate Immunity

Cell Type	Function
Basophils	Release inflammatory chemicals
Neutrophils	Phagocytosis, release toxic chemicals, NETs
Eosinophils	Phagocytosis, combat parasites and allergies
Monocytes/Macrophages	Phagocytosis, antigen presentation
NK Cells	Non-phagocytic killing of infected/cancerous cells

Additional info: Academic context and expanded explanations have been added to clarify mechanisms and provide examples for each topic.