Innate Immunity: The First Lines of Host Defense

Innate Immunity

Innate vs. Adaptive Immunity

The immune system protects the body from pathogens through two main branches: innate immunity and adaptive immunity. Innate immunity provides immediate, non-specific defense, while adaptive immunity is slower but highly specific and has memory.

• Innate Immunity: Present in all eukaryotes; responds immediately to pathogens without antigen specificity or memory.

• Adaptive Immunity: Found only in vertebrates; takes 4–7 days to respond, is antigen-specific, and generates immunological memory.

Feature	Innate Immunity	Adaptive Immunity
Response time	Immediate	4–7 days
Presence in organisms	All eukaryotes	Only vertebrates
Antigen specificity	No	Yes
Memory	No	Yes

Three Lines of Immune Defense

Overview of the Three Lines

The immune system is organized into three lines of defense, each with distinct roles in protecting the host from infection.

1. First-line (Barrier) Defenses: Prevent pathogen entry through mechanical, chemical, and physical barriers.

2. Second-line Defenses: Cellular and molecular responses that act if pathogens bypass barriers.

3. Third-line (Adaptive) Defenses: Antigen-specific responses with memory (covered in Chapter 12).

First-Line Defenses

• Mechanical Barriers: Physically remove pathogens via flushing, rinsing, or trapping (e.g., tears, mucus, cilia).

• Chemical Barriers: Destroy or inhibit microbes using substances like lysozyme, antimicrobial peptides, and acidic pH.

• Physical Barriers: Block pathogen entry with structures such as skin and tightly packed epithelial cells.

Example: The skin acts as a physical barrier, while tears contain lysozyme, a chemical barrier that breaks down bacterial cell walls.

Second-Line Defenses

• Cellular Defenses: Involve leukocytes (white blood cells) such as neutrophils, macrophages, dendritic cells, and natural killer (NK) cells.

• Molecular Defenses: Include cytokines, the complement system, and iron-binding proteins.

The Lymphatic System

Organization and Function

The lymphatic system is a fluid transport network that filters lymph and facilitates immune cell movement throughout the body.

• Lymph Nodes: Main filtering centers and sites of immune response activation.

• Spleen: Filters blood and helps mount immune responses.

• MALT (Mucosa-Associated Lymphoid Tissue): Protects mucosal surfaces.

• Thymus: Site of T cell maturation.

• Bone Marrow: Site of hematopoiesis (blood cell formation).

Example: Swollen lymph nodes during infection indicate active immune responses.

Leukocytes (White Blood Cells)

Main Types and Functions

• Granulocytes:

  • Neutrophils: Phagocytes that rapidly respond to infection.

  • Eosinophils: Combat parasites and mediate allergic responses.

  • Basophils: Involved in allergy and parasite defense.

  • Mast Cells: Tissue-resident cells important in allergy and inflammation.

• Agranulocytes:

  • Monocytes: Circulate in blood; differentiate into macrophages for phagocytosis.

  • Dendritic Cells: Antigen-presenting cells that activate adaptive immunity.

  • Lymphocytes: Include NK cells (innate immunity) and B/T cells (adaptive immunity; see Chapter 12).

Clinical Application: Leukocyte differentials help diagnose infections (e.g., increased neutrophils suggest bacterial infection).

Molecular Defenses

Cytokines

• Chemokines: Attract white blood cells to infection sites.

• Interleukins: Regulate inflammation and immune responses.

• Interferons: Induce antiviral states in cells.

• Tumor Necrosis Factors (TNFs): Promote inflammation and can trigger cell death.

Iron-Binding Proteins

• Limit pathogen growth by sequestering iron (e.g., transferrin, lactoferrin).

Complement System

• Composed of a series of proteins activated by three pathways: classical, alternative, or lectin.

• Leads to opsonization (enhanced phagocytosis), cytolysis (cell lysis), and inflammation.

Example: Complement activation can directly lyse bacterial cells or tag them for phagocytosis.

Inflammation and Fever

Inflammation

• Occurs in three phases: vascular changes, leukocyte recruitment, and resolution.

• Main goals: recruit immune defenses, limit pathogen spread, and promote tissue repair.

• Cardinal signs: redness, pain, heat, swelling, and loss of function.

Fever

• Systemic response triggered by pyrogens (fever-inducing substances).

• Enhances phagocytosis, interferon activity, and tissue repair.

• Low-grade fever is generally protective; high fever can be dangerous.

Example: Fever during infection helps the body fight pathogens but must be monitored to avoid complications.

Additional info: Adaptive immunity (third line) will be discussed in detail in Chapter 12, including the roles of B and T lymphocytes, antigen specificity, and immunological memory.