Innate Immunity: Nonspecific Lines of Defense

Overview of Nonspecific Defenses

The body’s nonspecific defenses, also known as innate immunity, provide immediate protection against pathogens. These defenses do not target specific microbes but instead offer broad protection through physical, chemical, and cellular mechanisms.

• Innate resistance: The natural ability of the body to resist infection through nonspecific mechanisms.

• Physical barriers: Skin and mucous membranes act as the first line of defense.

• Chemical barriers: Substances like lysozyme, sebum, and dermcidins inhibit microbial growth.

• Cellular defenses: Leukocytes and phagocytes play key roles in eliminating pathogens.

Skin and Mucous Membranes

The skin and mucous membranes form the primary physical barriers against infection.

• Skin: Composed of the epidermis and dermis. The epidermis is tightly packed and contains keratin, making it resistant to microbial invasion.

• Sebaceous glands: Produce sebum, which lowers skin pH and inhibits bacteria.

• Dermcidins: Antimicrobial peptides produced by sweat glands.

• Mucous membranes: Line body cavities; goblet cells produce mucus to trap microbes.

• Lacrimal glands: Secrete tears containing lysozyme, an enzyme that destroys bacterial cell walls.

Leukocytes and Phagocytosis

Leukocytes (white blood cells) are essential for innate immunity. They are classified as granulocytes or agranulocytes and participate in phagocytosis and other defense mechanisms.

• Granulocytes: Include neutrophils, eosinophils, and basophils.

• Agranulocytes: Include monocytes (which become macrophages), lymphocytes, and dendritic cells.

• Phagocytes: Cells that engulf and digest pathogens; main types are neutrophils, macrophages, and dendritic cells.

• Phagocytosis: The process by which phagocytes ingest and destroy microbes. Steps include adherence, ingestion, digestion, and elimination.

• Diapedesis: Movement of leukocytes out of blood vessels into tissues.

• Margination: The process by which leukocytes adhere to blood vessel walls near infection sites.

Phagocytosis Steps

1. Adherence: Phagocyte binds to pathogen.

2. Ingestion: Pathogen is engulfed into a phagosome.

3. Digestion: Phagosome fuses with lysosome; enzymes degrade pathogen.

4. Elimination: Debris is expelled from the cell.

Pathogen Associated Molecular Patterns (PAMPs) and Toll-like Receptors (TLRs)

PAMPs are molecular structures found on pathogens, recognized by TLRs on host cells, triggering immune responses.

• PAMPs: Conserved microbial molecules (e.g., lipopolysaccharide, peptidoglycan).

• Toll-like Receptors (TLRs): Proteins on immune cells that detect PAMPs and activate defense mechanisms.

• Cytokines: Signaling proteins released upon TLR activation, promoting inflammation and immune cell recruitment.

The Complement System

The complement system is a group of proteins that enhance immune responses. Activation occurs via three pathways and leads to pathogen elimination.

• Classical pathway: Triggered by antibodies bound to antigens.

• Alternative pathway: Activated directly by pathogen surfaces.

• Cascades: Sequential activation of complement proteins.

• Membrane attack complex (MAC): Forms pores in pathogen membranes, causing cell lysis.

• Opsonization: Complement proteins (opsonins) coat pathogens, enhancing phagocytosis.

Complement Activation and Bacterial Elimination

• Activation leads to inflammation, opsonization, and direct killing of bacteria.

• Complement proteins recruit phagocytes and increase vascular permeability.

Interferons and Antiviral Defense

Interferons are cytokines that limit viral infections by inducing antiviral states in cells.

• Interferon α and β: Produced by virus-infected cells; stimulate production of antiviral proteins in neighboring cells.

• Antiviral proteins inhibit viral replication and spread.

Inflammation

Inflammation is a localized response to infection or injury, characterized by redness, heat, swelling, and pain.

• Hallmarks: Redness, heat, swelling (edema), pain.

• Initiation: Triggered by tissue damage or infection.

• Chemicals involved: Bradykinin, prostaglandins, histamine, leukotrienes.

• Progression: Vasodilation, increased permeability, recruitment of leukocytes.

• Completion: Removal of pathogens, tissue repair.

Role of Inflammatory Chemicals

• Bradykinin: Increases vascular permeability and pain.

• Prostaglandins: Enhance effects of histamine and bradykinin.

• Histamine: Causes vasodilation and increased permeability.

• Leukotrienes: Attract leukocytes and increase permeability.

Fever

Fever is a systemic response to infection, characterized by elevated body temperature.

• Pyrogens: Substances that induce fever; can be produced by pathogens or host cells.

• Fever enhances immune function and inhibits microbial growth.

Key Vocabulary Table

The following table summarizes important terms related to innate immunity:

Summary

• Innate immunity provides immediate, nonspecific protection against pathogens.

• Physical and chemical barriers, cellular defenses, and molecular mechanisms work together to prevent infection.

• Understanding these components is essential for recognizing how the body responds to microbial threats.