Chapter 12: Host Defenses – Overview and Innate Defenses (Microbiology Fundamentals)

Host Defenses: Overview

Three Lines of Host Defenses

The human body employs a multilevel network of defenses against microbial invasion, classified into three lines of defense: innate (nonspecific) and adaptive (specific) mechanisms.

• First Line of Defense: Physical and chemical barriers that block invasion at the portal of entry. These are general in action and limit access to internal tissues.

• Second Line of Defense: Internalized system of protective cells and fluids, including inflammation and phagocytosis. Acts rapidly at both local and systemic levels once the first line is breached.

• Third Line of Defense: Acquired immunity, developed on an individual basis as each foreign substance is encountered by lymphocytes. Produces unique protective substances and provides long-term immunity.

Flowchart: Major Components of Host Defenses

Host defenses are divided into innate (first and second lines) and adaptive (third line) responses. The first line includes physical, chemical, and microbiota barriers; the second line includes phagocytosis, inflammation, fever, and antimicrobial proteins; the third line involves B cells, T cells, and antibodies.

Mandate of the Immune System

Functions

• Surveillance: Monitoring the body for foreign invaders.

• Recognition: Identifying foreign material via cell surface markers.

• Destruction: Eliminating entities deemed foreign, primarily through phagocytosis.

Immune Function

Self vs. Nonself Recognition

White blood cells (WBCs) constantly patrol the body, distinguishing self cells from nonself (foreign) material. This ability is central to immune function; errors can result in autoimmune disorders.

Markers and Antigens

• Markers (Antigens): Molecules (proteins/sugars) on cell surfaces that allow immune cells to identify threats.

• Phagocytosis: The most common method of destruction for marked cells.

PAMPs and PRRs

• Pathogen-Associated Molecular Patterns (PAMPs): Shared markers among microbes, not present in mammals.

• Pattern Recognition Receptors (PRRs): Host cell receptors that recognize PAMPs, crucial for innate immunity.

The Immune System as a Network

The immune system is a diffuse network of cells and fluids permeating every organ and tissue, promoting surveillance and recognition of harmful substances.

Tissues, Organs, and Cells in Immunity

Mononuclear Phagocyte System (MPS)

• Connective tissue network (reticulum) supporting and interconnecting cells throughout the body.

• Phagocytic cells (macrophages, dendritic cells) enmeshed in this network, in direct contact with tissue cells and extracellular fluid.

• Blood and lymphatic capillaries penetrate tissues, allowing diffusion of immune cells and chemicals.

Locations of MPS

• Thymus (WBC maturation)

• Lymph nodes

• Tonsils

• Spleen

• Lymphoid tissue of gut and respiratory tract

Lymphatic System

Vessels and Fluids

• Compartmentalized network transporting lymph through vessels and nodes, draining back to the circulatory system.

• Lymphatic fluid: Plasma-like liquid formed from blood components diffusing into lymphatic capillaries, transporting WBCs, fats, debris, and infectious agents.

Functions

• Returns extracellular fluid to circulation

• Acts as a drain-off system for inflammation

• Provides surveillance, recognition, and protection via lymphocytes, phagocytes, and antibodies

Lymphatic Vessels

• Thin-walled, accompany blood capillaries, extend throughout the body except CNS, bone, placenta, thymus

• Walls permeable to extracellular fluid

• High numbers in hands, feet, breast areola

• Lymph flows from extremities to heart, moved by skeletal muscle contraction

Lymphatic Organs

• Primary: Red bone marrow (birth/maturation of B cells), Thymus (T cell maturation)

• Secondary: Lymph nodes, spleen, associated lymphoid tissues

Red Bone Marrow

• Site of blood cell production in flat and long bones

• B lymphocytes mature here before migrating to secondary organs

Thymus

• Embryonic origin, site of T cell maturation and specificity development

• Mature T cells migrate to lymph nodes and spleen

Lymph Nodes

• Small, bean-shaped, clustered along lymphatic channels and large blood vessels

• Aggregations in armpit (axillary), groin (inguinal), neck (cervical)

• Enlargement indicates immune response

Spleen

• Filters blood, removes worn-out RBCs, and pathogens for phagocytosis

• Storehouse of blood for emergencies

• Children without spleen are severely immunocompromised

Associated Lymphoid Tissues

• Bundles of lymphocytes beneath skin and mucosa (SALT, MALT)

• Tonsils, breasts (during pregnancy/lactation), gut-associated lymphoid tissue (GALT: appendix, lacteals, Peyer's patches)

The Blood and Hematopoiesis

Components

• Blood cells: Formed elements in plasma

• Plasma: Clear, yellowish fluid

• Serum: Plasma from clotted blood, used in immune testing

• Hematopoiesis: Production of blood cells from stem cells in bone marrow

Stem Cells and Leukocytes

• Stem cells differentiate into RBCs, platelets, and WBCs

• WBCs (leukocytes) are granulocytes or agranulocytes, vital for immunity

Cytokines: Cell Communication

Definition and Function

• Small active molecules regulating, stimulating, suppressing, and controlling cell development, inflammation, and immunity

• Produced by monocytes, macrophages, lymphocytes, fibroblasts, mast cells, platelets, endothelial cells

Cytokine Types and Examples

Type	Examples	Source	Target
Pro-inflammatory	Interleukin-1 (IL-1), TNF-β	Macrophages, T cells	B cells, T cells, phagocytes, tumor cells
Anti-inflammatory	Interleukin-10 (IL-10)	T cells	B cells, macrophages
Vasodilators	Serotonin, Histamine	Platelets, mast cells	Blood vessels, sensory neurons
Growth factors	Interleukin-7 (IL-7), Erythropoietin	Bone marrow cells, endothelial cells	Stem cells

First Line of Defense

Physical and Chemical Barriers

• Skin: Stratum corneum (impervious, waterproof), sloughing of outer layers, desquamation of hair shaft and follicle cells, sweat flushing

• Mucous Membranes: Mucous coat, blinking/tears, saliva flow, vomiting, defecation

• Respiratory Tract: Nasal hair, mucus flow, ciliated epithelium, sneeze/cough reflex

• Genitourinary Tract: Urine flow, bladder emptying, vaginal secretions

Human Microbiome

• Blocks pathogen access to epithelial surfaces

• Competes for nutrients, alters local pH

• Disruption can lead to diseases like Crohn's and ulcerative colitis

Nonspecific Chemical Defenses

• Sebaceous secretions, sweat (acidic, antimicrobial)

• Lysozyme in tears/saliva (hydrolyzes peptidoglycan)

• Acidic pH and fatty acids in skin

• Hydrochloric acid in stomach, digestive juices, bile

• Protective vaginal pH maintained by normal biota

Second Line of Defense

Major Categories

• Phagocytosis

• Inflammation

• Fever

• Antimicrobial proteins

Phagocytosis

• Phagocytes survey tissues, ingest/eliminate microbes, and read antigens

• Types: Neutrophils (early responders, pus formation), Monocytes (transform into macrophages/dendritic cells), Histiocytes (tissue-resident macrophages)

Steps of Phagocytosis

1. Chemotaxis

2. Ingestion

3. Phagolysosome formation

4. Destruction

5. Excretion

Inflammation

• Signs: Rubor (redness), Calor (warmth), Tumor (swelling), Dolor (pain), Loss of function

• Functions: Mobilize immune components, repair tissue, destroy/block microbes

• Stages: Injury/immediate reactions, vascular reactions, edema/pus formation, resolution/scar formation

Diapedesis and Chemotaxis

• Diapedesis: WBC migration out of blood vessels into tissues

• Chemotaxis: Directed migration in response to chemical signals

Benefits of Edema and Leaky Vessels

• Dilutes toxins, traps microbes, aggregates neutrophils for phagocytosis

• Pus formation by pyogenic bacteria (e.g., Streptococcus, Staphylococcus)

Fever

• Elevated body temperature, common in infection

• Pyrogens reset hypothalamic thermostat (exogenous: microbial products; endogenous: cytokines like IL-1, TNF)

• Benefits: Inhibits microbe multiplication, reduces iron availability, increases metabolism and immune reactions

Antimicrobial Host-Derived Products

• Interferons: Small proteins produced by WBCs and tissue cells, regulate immunity, inhibit viral replication, and have tumor suppressor effects

• Complement System: Over 30 blood proteins acting in a cascade to destroy microbes via membrane attack complex (MAC)

• Antimicrobial Peptides: Short proteins (defensins, etc.) that insert into microbial membranes, causing lysis

Complement Pathways

• Classical: Initiated by antibody binding to microbes

• Alternative: Initiated by foreign antigens, faster response

Stages of Complement Cascade

1. Initiation (C3 hydrolysis)

2. Activation and cascade (C5 cleavage)

3. Polymerization (MAC formation)

4. Membrane attack (cell lysis)

Interferon Overview

• Produced in response to viral infection

• Induces antiviral proteins, inhibits viral replication, and modulates immune responses

Summary Table: Major Host Defense Mechanisms

Line of Defense	Main Components	Examples
First	Physical/Chemical Barriers	Skin, mucous membranes, microbiota, secretions
Second	Cellular/Protein Defenses	Phagocytes, inflammation, fever, complement, interferon
Third	Adaptive Immunity	B cells, T cells, antibodies

Additional info: These notes expand on the provided slides with definitions, examples, and mechanisms for each major topic, suitable for exam preparation in a college-level microbiology course.