Innate Immune System: Structure, Function, and Response

Innate Immune System

General Purpose, Components, and Organization

The innate immune system is the body's first line of defense against pathogens, providing immediate, non-specific responses. It consists of physical, chemical, and biological barriers, as well as cellular and soluble components that recognize and respond to common pathogen-associated molecular patterns (PAMPs).

• Physical Barriers: Skin, mucous membranes, cilia, tears

• Chemical Barriers: Acid, enzymes (pepsin, lysozyme), bile, defensins

• Biological Barriers: Normal flora

• Cellular Components: Phagocytes (macrophages, neutrophils), dendritic cells, innate lymphocytes

• Soluble Components: Complement proteins, cytokines, chemokines

Comparison of Innate and Adaptive Immune Systems

The immune system is divided into two overlapping compartments: innate and adaptive immunity. The innate system responds immediately and non-specifically, while the adaptive system is highly specific, slower to respond, and generates immunological memory.

• Innate Immunity: Immediate, non-specific, no memory, recognizes general patterns

• Adaptive Immunity: Delayed (3-5 days), highly specific, generates memory, recognizes specific antigens

Detection and Recognition of Pathogens

Pathogens are detected by the immune system through pattern recognition. Cells and soluble factors recognize PAMPs using pattern recognition receptors (PRRs), including Toll-like receptors (TLRs) and NOD-like receptors (NLRs).

• PAMPs: Molecules expressed by pathogens but not by host cells (e.g., lipopolysaccharide, flagellin, viral RNA)

• PRRs: Cellular and soluble receptors that bind PAMPs and activate immune responses

Pattern Recognition Receptors (PRRs) and Toll-Like Receptors (TLRs)

PRRs are found on the surface and inside immune cells. TLRs are a major family of PRRs, each recognizing specific microbial molecules. Some TLRs are located on the cell surface, others in intracellular compartments.

• Cell Surface TLRs: Detect extracellular pathogens

• Intracellular TLRs: Detect intracellular pathogens (e.g., viruses)

• NOD-Like Receptors: Detect intracellular bacterial components

TLR	PAMP (Microbial Molecule)
TLR1	Bacterial lipopeptides and certain proteins in multicellular parasites
TLR2	Peptidoglycan, lipoteichoic acid (Gram-positive cell wall), cell wall of yeast
TLR4	Lipid A in LPS (Gram-negative bacteria)
TLR5	Flagellin (bacterial flagella)
TLR6	Bacterial lipopeptides, lipoteichoic acid, cell wall of yeast
TLR10	Unknown component of influenzaviruses
TLR3	Double-stranded RNA (viruses)
TLR7	Single-stranded viral RNA
TLR8	Single-stranded viral RNA
TLR9	Unmethylated cytosine-guanine pairs of viral and bacterial DNA

Complement System

The complement system is a cascade of plasma proteins that can be activated by three pathways: classical (antibody-bound), alternative (direct microbe binding), and lectin (lectin binding to microbial carbohydrates). Activation leads to the formation of the membrane attack complex (MAC), which lyses pathogens, and to opsonization and recruitment of immune cells.

• Classical Pathway: Activated by antibody-antigen complexes

• Alternative Pathway: Activated directly by microbial surfaces

• Lectin Pathway: Activated by lectin binding to microbial carbohydrates

• MAC: Forms pores in pathogen membranes, leading to cell lysis

Innate Immune Response: Sequence of Events

Innate immune responses follow a characteristic sequence: recognition, alarm, inflammation, cell recruitment, and effector functions.

• Recognition: Detection of pathogens by PRRs

• Alarm: Release of cytokines and chemokines to activate and localize immune cells

• Inflammation: Vasodilation, increased permeability, extravasation of white blood cells, stimulation of pain receptors

• Cell Recruitment: Phagocytes, neutrophils, natural killer cells, eosinophils, basophils, monocytes/macrophages

• Effector Functions: Phagocytosis, cytotoxicity, production of reactive oxygen species, antimicrobial peptides, destructive enzymes

Phagocytic Cells and Their Functions

Phagocytic cells, including macrophages, neutrophils, and dendritic cells, ingest and destroy pathogens. They may use lysosomes or activate a respiratory burst to generate reactive oxygen and nitrogen species.

• Macrophages: Patrol tissues, ingest pathogens, key in both innate and adaptive immunity

• Neutrophils: Phagocytize bacteria and debris, form pus, use NETs

• Dendritic Cells: Professional antigen-presenting cells, bridge innate and adaptive immunity

Lymphatic System and Hematopoiesis

The lymphatic system drains excess fluid and proteins from tissues and transports them to the blood. It includes primary and secondary lymphoid tissues, where lymphocytes mature and interact with pathogens. Hematopoiesis is the development of blood cells from pluripotent stem cells in the bone marrow.

• Primary Lymphoid Tissue: Thymus, bone marrow

• Secondary Lymphoid Tissue: Lymph nodes, MALT, spleen, tonsils, adenoids

• Lymph Node: Site for immune cell interaction and activation

• Hematopoiesis: Formation of all blood cells, including leukocytes

Leukocyte Lineages

Leukocytes (white blood cells) are divided into granulocytes, monocytes/macrophages, dendritic cells, and lymphocytes, each with distinct roles in immunity.

• Granulocytes: Neutrophils, basophils/mast cells, eosinophils

• Monocytes/Macrophages: Circulate in blood, mature in tissues, phagocytosis

• Dendritic Cells: Antigen presentation, initiate adaptive response

• Lymphocytes: B cells (antibody production), T cells (helper and cytotoxic), natural killer cells (innate cytotoxicity)

Summary Table: Skin vs. Mucous Membranes

The skin and mucous membranes serve as the first line of defense, each with unique structural and functional properties.

	Skin	Mucous Membrane
Number of Cell Layers	Many	One to a few
Cells Tightly Packed?	Yes	Yes
Cells Dead or Alive?	Outer layers: dead; inner layers: alive	Alive
Mucus Present?	No	Yes
Relative Water Content	Dry	Moist
Lysozyme Present?	Yes	With some
Defensins Present?	Yes	Yes
Sebum Present?	Yes	No
Cilia Present?	No	Trachea, uterine tubes
Constant Shedding and Replacement of Cells?	Yes	Yes

Summary Table: Secretions and Activities Contributing to First Line of Defense

Secretion/Activity	Function
Saliva	Washes microbes from teeth, gums, tongue; contains lysozyme
Stomach acid	Digests and/or inhibits microorganisms
Bile	Inhibits microorganisms
Intestinal secretions	Digests and/or inhibits microorganisms
Defecation	Eliminates microorganisms
Urine	Contains lysozyme; acidity inhibits microorganisms
Vaginal secretions	Acidity inhibits microorganisms
Menstrual flow	Cleanses uterus and vagina
Blood flow	Removes microorganisms from wounds
Coagulation	Prevents entrance of many pathogens

Practice and Application

Understanding the innate immune system is essential for recognizing how the body responds to infection and for diagnosing immune-related diseases. The presence and activity of specific immune cell types in tissues can indicate immune system activity and disease states.

Example: Increased neutrophil recruitment is a hallmark of acute bacterial infection, while eosinophil accumulation is associated with parasitic infections and allergies.

Additional info: The innate immune system is evolutionarily conserved and forms the basis for many therapeutic interventions, including vaccines and immunomodulatory drugs.