Innate Immunity: Nonspecific Defenses of the Host (Chapter 16) – Study Notes

Innate Immunity: Nonspecific Defenses of the Host

Overview of Innate vs. Adaptive Immunity

The immune system protects the body from pathogens through two main strategies: innate (nonspecific) and adaptive (specific) immunity. Innate immunity provides immediate, general defense mechanisms, while adaptive immunity develops more slowly and targets specific pathogens.

• Innate Immunity: Present from birth, responds rapidly to a broad range of pathogens, does not confer long-lasting immunity.

• Adaptive Immunity: Develops after exposure to specific antigens, involves B and T lymphocytes, provides memory for faster future responses.

Example: Skin and mucous membranes are part of innate immunity, while antibody production is part of adaptive immunity.

First Line of Defense: Physical, Chemical, and Biological Barriers

Physical Barriers

Physical barriers prevent the entry of pathogens into the body.

• Skin: Acts as a tough, impermeable barrier; shedding of skin removes microbes.

• Mucous Membranes: Line the respiratory, gastrointestinal, and genitourinary tracts; mucus traps microbes.

• Mechanical Actions: Cilia in the respiratory tract move mucus and trapped particles out; tears, saliva, and urine flush out microbes.

Chemical Barriers

Chemical factors inhibit or destroy pathogens.

• Lysozyme: Enzyme found in tears, saliva, and mucus; breaks down bacterial cell walls.

• Lactoferrin & Transferrin: Proteins that bind iron, making it unavailable to microbes.

• Defensins: Small antimicrobial peptides that disrupt microbial membranes.

• Acidity: Low pH of stomach (gastric juice) and skin inhibits microbial growth.

Normal Flora (Microbiota)

Normal flora are non-pathogenic microorganisms that reside on body surfaces and provide protection.

• Competitive Exclusion: Normal flora outcompete pathogens for nutrients and attachment sites.

• Production of Inhibitory Substances: Some normal flora produce bacteriocins or acids that inhibit pathogens.

Example: Lactobacillus in the vagina produces lactic acid, lowering pH and preventing pathogen growth.

Second Line of Defense: Cellular Components of Blood

Blood Components and Their Roles

• Erythrocytes (Red Blood Cells): Transport oxygen; do not play a direct role in immunity.

• Platelets: Involved in blood clotting; help prevent pathogen entry through wounds.

• Leukocytes (White Blood Cells): Main cells of the immune system; some are phagocytic.

Types of Leukocytes

• Neutrophils (PMNs): Most abundant; highly phagocytic; first responders to infection.

• Basophils: Release histamine; involved in inflammation and allergic responses.

• Eosinophils: Combat parasites; involved in allergic reactions.

• Monocytes/Macrophages: Monocytes circulate in blood; become macrophages in tissues; professional phagocytes.

• Dendritic Cells: Phagocytic; bridge innate and adaptive immunity by presenting antigens.

• B and T Lymphocytes: Key cells of adaptive immunity; do not participate in innate immunity.

• Natural Killer (NK) Cells: Destroy infected or abnormal cells by releasing cytotoxic granules.

Professional Phagocytes: Neutrophils, macrophages, and dendritic cells.

Pattern Recognition and Signaling Molecules

Toll-Like Receptors (TLRs)

Toll-like receptors are proteins on immune cells that recognize pathogen-associated molecular patterns (PAMPs) such as bacterial lipopolysaccharide or viral RNA.

• Function: Detect conserved microbial structures and trigger immune responses.

Cytokines

Cytokines are small proteins released by cells that regulate immune responses.

• Roles: Mediate communication between cells, promote inflammation, recruit immune cells, and regulate cell growth.

Example: Interleukin-1 (IL-1) induces fever and inflammation.

Phagocytosis

Process and Stages of Phagocytosis

Phagocytosis is the process by which certain cells ingest and destroy microbes and debris.

1. Chemotaxis: Phagocytes are attracted to infection sites by chemical signals.

2. Adherence: Phagocyte attaches to the microbe (often enhanced by opsonins such as C3b).

3. Ingestion: Microbe is engulfed into a phagosome.

4. Digestion: Phagosome fuses with lysosome to form a phagolysosome; enzymes digest the microbe.

5. Exocytosis: Indigestible material is expelled from the cell.

Cells Involved in Phagocytosis

• Neutrophils

• Macrophages

• Dendritic cells

Microbial Evasion of Phagocytosis

• Capsules: Streptococcus pneumoniae uses a capsule to avoid phagocytosis.

• Inhibition of Phagolysosome Fusion: Mycobacterium tuberculosis prevents fusion, surviving inside phagocytes.

• Survival in Phagolysosome: Some bacteria resist digestion.

Opsonization: The process by which opsonins (e.g., C3b) coat microbes, enhancing phagocytosis.

Inflammatory Response

Signs and Causes of Inflammation

Inflammation is a localized response to infection or injury, aiming to contain and eliminate pathogens and initiate tissue repair.

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

• Causes: Infection, tissue injury, or immune reactions.

Triggers and Steps of Inflammation

1. Tissue Damage: Releases chemical mediators (e.g., histamine, cytokines).

2. Vasodilation: Blood vessels widen, increasing blood flow (redness, heat).

3. Increased Vascular Permeability: Fluid and immune cells leave blood vessels (swelling).

4. Phagocyte Migration (Diapedesis): Neutrophils and monocytes move through vessel walls to the site (chemotaxis).

5. Pus Formation: Accumulation of dead cells and microbes.

6. Tissue Repair: Damaged tissue is replaced or healed.

Cells Involved: Neutrophils arrive first, followed by monocytes/macrophages.

Key Terms: Vasodilation (widening of blood vessels), Diapedesis (movement of cells out of blood vessels).

Fever

Fever is a systemic increase in body temperature, often in response to infection.

• Mechanism: Pyrogens (e.g., IL-1) are released by phagocytes, signaling the hypothalamus to raise body temperature.

• Benefits: Inhibits growth of some microbes, enhances immune responses, speeds up tissue repair.

Chemical Responses to Invading Organisms

Interferons

Interferons are cytokines produced by virus-infected cells that help protect neighboring cells from viral infection.

• Types: Alpha, beta, and gamma interferons.

• Induction: Produced in response to viral infection.

• Effectiveness: Limit viral spread, activate immune cells.

Complement System

The complement system is a group of plasma proteins that enhance immune responses through a cascade of activation.

• Activation Pathways:

  • Classical pathway

  • Alternative pathway

  • Lectin pathway

Consequences/Functions of Complement Activation

Function	Description	Key Complement Proteins
Opsonization	Coating of microbes to enhance phagocytosis	C3b
Inflammation	Attraction and activation of immune cells	C3a, C5a
Cytolysis (Membrane Attack Complex)	Formation of pores in microbial membranes, leading to cell lysis	C5b, C6, C7, C8, C9

Example: C3b acts as an opsonin, C5a is a chemoattractant, and the membrane attack complex (C5b-C9) lyses bacteria.

Additional info: The complement system bridges innate and adaptive immunity and is tightly regulated to prevent host damage.