Week 6: Body's Defences 1

Learning Objectives

Students will be able to use anatomical position, directional terms, and body-compartment language to describe where protective tissues and membranes are located.
Students will be able to distinguish mucous from serous membranes and explain how epithelial tissues protect open passages and closed body cavities.
Students will be able to explain the structure of thin and thick skin, the barrier functions of the integumentary system, and the stages of tissue remodelling after injury.
Students will be able to distinguish innate from adaptive immunity and identify the roles of granulocytes, phagocytes, antigen-presenting cells, and major chemical barriers.
Students will be able to explain the antibody barrier, including immunoglobulin classes, antibody actions, vaccination versus immunization, and why waning immunity can require boosters.
Students will be able to describe granuloma formation as a strategy for isolating persistent pathogens that cannot be removed immediately.

Understanding the Body's Defense Systems

Your body is like a fortress with multiple layers of defense protecting you from invaders like bacteria and viruses. Just as a castle has walls, gates, guards, and moats, your body has barriers, chemical weapons, and specialized soldiers that work together to keep you healthy.

The first line of defense is your skin and the membranes lining your internal surfaces. Think of your skin as a thick castle wall made of bricks (cells) held together with mortar (proteins). This wall keeps most invaders out. Your skin also produces an acid mantle - like pouring acid in a moat around the castle - that kills many bacteria. It also produces special proteins called defensins that poke holes in bacterial cell walls, making them burst like water balloons.

Inside your body, you have different compartments separated by membranes, like rooms in a house. These membranes are made of epithelial tissue - sheets of cells that line surfaces and protect what's underneath. There are different types: some are flat like floor tiles (squamous), some are tall like columns (columnar), and some have tiny hairs called cilia that sweep away invaders. These membranes line your lungs, digestive tract, and other passages, forming barriers that pathogens must breach to cause infection.

When invaders do get past these barriers, your immune system sends in soldiers called white blood cells or leukocytes. These are like security guards that patrol your body through the bloodstream. Some are neutrophils, which are like quick-response troops that arrive first at infection sites and eat invaders. Others are macrophages, which are like big hungry Pac-Man cells that swallow and digest pathogens. Then there are lymphocytes - B cells and T cells - which are like special intelligence agents that remember specific invaders and mount targeted attacks.

Your body also uses chemical weapons. Lysozyme is an enzyme found in tears, saliva, and mucus that breaks down bacterial cell walls. Your stomach produces hydrochloric acid so strong it can dissolve metal - this kills most germs you swallow. These are all part of your innate immunity - defenses you're born with that work immediately against any threat.

Advanced: Immunology and Body Defenses

The body's defense systems represent a sophisticated multi-layered protection mechanism that has evolved to protect against pathogenic microorganisms while maintaining tolerance to commensal flora and self-antigens. These defenses are organized into physical/mechanical barriers, chemical barriers, and immune responses, which function in coordinated fashion to maintain homeostasis.

The anatomical organization of the body follows standardized terminology based on the anatomical position: standing upright, facing forward, arms at sides with palms forward. Key directional terms include anterior/posterior, medial/lateral, proximal/distal, and superficial/deep. The body is divided into compartments - the cranial cavity housing the brain, thoracic cavity containing the heart and lungs, abdominal cavity with digestive organs, and pelvic cavity - each protected by membranes.

Epithelial tissues form the primary barrier between the internal and external environments. The five main types include: simple squamous (single layer of flat cells for diffusion), simple cuboidal (cube-shaped cells for secretion and absorption), simple columnar (tall cells with microvilli or cilia for absorption and movement), stratified squamous (multiple layers for protection, keratinized in skin), and pseudostratified ciliated columnar (respiratory tract with mucus-producing goblet cells and cilia for particle removal).

The integumentary system (skin) serves as the body's largest organ and first line of defense. Structurally, it consists of the epidermis (keratinized stratified squamous epithelium), dermis (connective tissue with blood vessels, nerves, and immune cells), and hypodermis (subcutaneous fat layer). The epidermis contains five layers in thick skin: stratum corneum (dead keratinized cells), stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale (stem cell layer with melanocytes).

Skin functions include: protection (chemical, physical, biological barriers), thermoregulation (vasodilation/vasoconstriction, sweating), sensation (mechanoreceptors, thermoreceptors, nociceptors), vitamin D synthesis, and immune surveillance (Langerhans cells). The acid mantle (pH 4-6) inhibits pathogen growth while supporting commensal microbiota. Defensins and cathelicidins are antimicrobial peptides that disrupt microbial membranes.

📍 Anatomy, Compartments, and Membranes

Week 6a starts with the language used to describe where tissues and protective barriers are found. In the anatomical position, the body stands upright facing forward, arms at the sides, and palms facing forward. If the body lies face up it is supine; if it lies face down it is prone.

That shared language lets you describe barrier tissues accurately: anterior/posterior for front and back, medial/lateral for toward or away from the midline, proximal/distal for nearer or farther from a point of attachment, and superficial/deep for closer to or farther from the surface.

The body is organized into compartments that help separate and protect organs. The cranial cavity houses the brain, the vertebral canal houses the spinal cord, the thoracic cavity contains the heart and lungs, and the abdominopelvic cavity contains digestive, urinary, and reproductive organs.

Mucous membranes line passages open to the outside world, such as the respiratory, digestive, urinary, and reproductive tracts. They help defend the body by producing mucus, trapping particles, and supporting epithelial barriers. Serous membranes line closed cavities and cover organs within them. Their parietal and visceral layers, with serous fluid between them, reduce friction around moving organs such as the lungs, heart, and abdominal viscera.

Mucous membrane examples: respiratory tract, gastrointestinal tract, reproductive tract
Serous membrane examples: pleura, pericardium, peritoneum
Epithelial role: flat, cuboidal, columnar, stratified, and pseudostratified epithelia are placed where diffusion, secretion, absorption, movement, or protection is needed

🧴 Skin, Thin vs Thick Skin, and Tissue Remodelling

The integumentary system is more than a covering. It provides physical, chemical, and biological defense while also helping with sensation, temperature regulation, vitamin D production, excretion, and blood storage. Thick skin, found on the palms and soles, has five epidermal layers including the stratum lucidum. Thin skin covers most of the body and lacks that extra layer.

The skin's protective barrier depends on keratin, glycolipid-based waterproofing, the acid mantle, antimicrobial peptides such as defensins, and resident immune cells such as dendritic cells and macrophages. Normal microbiota also compete with pathogens at the surface.

Tissue remodelling matters because skin is repeatedly injured. After injury, repair moves through three linked stages described in the Week 6 source:

Inflammation: hemostasis, vasodilation, neutrophil entry, macrophage cleanup
Organization: granulation tissue, angiogenesis, fibroblast activity, collagen deposition
Regeneration: epithelial closure, collagen remodelling, scar maturation

Superficial damage can heal with little long-term change, but deeper injury may lead to fibrosis, scar formation, contracture, or the need for grafting if the dermis is extensively destroyed.

Thin skin: most of the body, hair follicles and sebaceous glands common
Thick skin: palms and soles, more epidermal layers, better abrasion resistance
Deep burns: loss of dermal structures reduces regenerative capacity and increases scarring risk

🛡️ Immune Recognition, Vaccination, and the Antibody Barrier

MHC class I molecules are found on almost all nucleated cells and present intracellular material to CD8 T cells. MHC class II molecules are mainly found on professional antigen-presenting cells such as dendritic cells, macrophages, and B cells, where they present ingested material to CD4 helper T cells.

Innate immunity responds quickly with barriers, phagocytes, inflammation, and chemical defenses. Adaptive immunity is slower at first, but it is specific and creates memory. This distinction explains why antibodies, B cells, and T cells become more effective after exposure to a pathogen or vaccine.

Vaccination is the administration of antigen-containing material. Immunization is the protective immune state that develops when the body responds to that material. In other words, vaccination is the input; immunization is the result if memory and protection are formed.

Active immunity develops when your own immune system responds and makes memory. Passive immunity is the transfer of ready-made antibodies, such as maternal antibodies or therapeutic immunoglobulin. Passive immunity acts quickly, but it fades because it does not create memory cells.

The antibody barrier depends on what antibodies do after binding antigen. Antibodies can neutralize toxins and viruses, agglutinate targets, opsonize pathogens for phagocytosis, and trigger complement activation. Over time, antibody levels can fall. That waning immunity is why some infections or vaccines require boosters to raise antibody titres and refresh immune memory.

🎥 Video Lectures

Week 6 Overview

Introduction to body defenses and immunity

Lecture Topics

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📄 Lecture Notes

Key Terms

Epithelial Tissue

Sheets of cells that cover body surfaces, line cavities, and form glands. Types include squamous (flat), cuboidal (cube-shaped), and columnar (tall), arranged in simple (single layer) or stratified (multiple layers) configurations.

Integumentary System

The skin and its appendages (hair, nails, glands), serving as the body's largest organ and primary physical barrier against environmental threats.

Keratin

A tough fibrous protein produced by keratinocytes that provides mechanical strength and waterproofing to the epidermis, hair, and nails.

Acid Mantle

The slightly acidic (pH 4-6) film on the skin surface created by sebum, sweat, and normal bacterial byproducts that inhibits pathogen growth.

Anatomical Position

The standard reference position for describing the body: standing upright, facing forward, arms at the sides, palms facing forward.

Mucous Membrane

A membrane lining body passages that open to the outside. It produces mucus that protects, lubricates, and traps particles and microbes.

Serous Membrane

A membrane lining closed body cavities and covering organs within them. Its parietal and visceral layers reduce friction with serous fluid.

Defensins

Antimicrobial peptides produced by epithelial cells and immune cells that insert into microbial membranes, creating pores that cause cell lysis.

Lysozyme

An enzyme discovered by Alexander Fleming that hydrolyzes peptidoglycan in bacterial cell walls, causing osmotic rupture. Found in tears, saliva, and secretions.

Leukocytes

White blood cells that mediate immune responses, including granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes).

Neutrophils

The most abundant granulocyte (50-70% of leukocytes), primarily functioning as phagocytes that arrive first at infection sites and release inflammatory mediators.

Macrophages

Large phagocytic cells derived from monocytes that engulf pathogens, present antigens, and release cytokines to coordinate immune responses.

Phagocytosis

The cellular process of engulfing and digesting foreign particles, pathogens, or cellular debris, primarily performed by neutrophils and macrophages.

Innate Immunity

Non-specific defense mechanisms present from birth, including physical barriers, phagocytic cells, inflammation, and complement proteins, providing immediate but generalized protection.

Adaptive Immunity

Antigen-specific defense involving B and T lymphocytes that develops over days but provides long-term immunological memory and targeted responses.

Antibodies

Y-shaped glycoproteins (immunoglobulins) produced by plasma cells that specifically bind antigens, marking them for destruction or neutralization.

Immunoglobulins

The five classes of antibodies: IgG (serum antibody), IgA (mucosal), IgM (early response), IgE (allergies), and IgD (B cell receptor).

Granuloma

An organized collection of macrophages, lymphocytes, and giant cells that walls off persistent pathogens or irritants that cannot be eliminated.

Goblet Cells

Mucus-producing cells interspersed in epithelial tissues, particularly abundant in respiratory and intestinal tracts, secreting mucin that hydrates to form protective mucus.

Cilia

Hair-like projections on epithelial cells that beat in coordinated waves to move mucus and trapped particles along epithelial surfaces.

Thin Skin

The skin covering most of the body. It lacks a stratum lucidum and commonly contains hair follicles and sebaceous glands.

Thick Skin

Skin found on the palms and soles. It has an extra epidermal layer, the stratum lucidum, and is specialized for abrasion resistance.

Cytokines

Small protein signaling molecules released by immune cells that regulate inflammation, cell activation, and communication between immune system components.

Immunization

The protected immune state that results when vaccination or infection successfully stimulates adaptive immunity and memory.

Waning Immunity

The gradual decline of antibody levels or protective immunity over time, which can make booster doses necessary.

Interactive Activity

End of Week Test

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