Animal Responses (HSC SSCE Biology): Revision Notes

Animal Responses

Introduction

Animals face constant exposure to a vast array of micro-organisms that could potentially cause disease. Complex multicellular organisms have evolved a multilayered system of defences to prevent infection and colonisation by pathogens. The animal immune system is highly organised, similar to an army, with different levels of defence and the ability to anticipate invasion by pathogens it has not yet encountered.

Lines of defence

The body's immune system consists of three organised levels of defence, each with specific roles in protecting against pathogens.

Innate immunity

Innate immunity is present at birth and genetically determined. It provides non-specific responses to pathogens and includes:

• First line of defence: Physical and chemical barriers to pathogen entry
• Second line of defence: Cellular responses including inflammation and phagocytosis

These defences act against any substance recognised as 'non-self' and threatening, whether living organisms or non-living materials such as splinters, dust, or transplanted organs.

Adaptive immunity

Adaptive immunity is the third line of defence. This is a specific defence mechanism consisting of specialised cells (lymphocytes) that activate if pathogens persist in their invasion. This will be covered in detail in Chapter 12.

Body organisation and the immune system

The tube model of the body

A useful way to understand the body's organisation is to think of it as a hollow tube or pipe:

• The inside tunnel (lumen) represents the digestive system, which contains bacteria including both beneficial microflora and potential pathogens like E. coli
• The outside of the tube is exposed to environmental micro-organisms
• The tube wall itself represents the body's tissues, which must remain sterile

Sterile compartments

Certain internal compartments must remain free from microbial contamination:

• Thoracic cavity
• Cranial cavity
• Abdominal cavity
• Blood vessels of the cardiovascular system

Components of the immune system

Antigens

An antigen is any molecule that the body recognises as foreign and that triggers an immune response.

• Body cells have 'marker' molecules on their surface identifying them as 'self'
• Pathogens have different chemical markers (antigens) recognised as 'non-self'
• Antigens can come from pathogens, foreign cells, cell fragments, protein debris, bacterial toxins, or snake venom

The lymphatic system

The lymphatic system plays a crucial role in the body's defence. It consists of:

• Lymph (milky fluid derived from tissue fluid)
• Lymph vessels (one-way drainage system)
• Lymph nodes (filter pathogens and debris)
• Lymphoid organs: thymus, spleen, tonsils, adenoids

Function: As blood circulates, plasma moves into tissues as tissue fluid, then enters lymph vessels. Lymph flows through lymph nodes where pathogens, cellular debris, and cancer cells are filtered out before the cleansed fluid returns to the blood via the thoracic duct.

White blood cells (leucocytes)

White blood cells play pivotal roles in innate and adaptive immune responses. They differ in:

• Size
• Presence of granules in cytoplasm (granular or agranular)
• Colour of granules and cytoplasm (eosinophilic, basophilic)
• Shape of nucleus (lobed or mononuclear)

Major types of white blood cells and their functions:

Cell Type	Characteristics	Location
Neutrophil	Most common at infection sites; releases toxins killing bacteria and fungi; recruits other immune cells	Migrates from blood vessels into tissues
Eosinophil	Releases toxins that kill bacteria and parasites	Circulates in blood, migrates to tissues
Basophil	Defends against parasites; releases histamines causing inflammation; involved in allergic reactions	Circulates in blood, migrates to tissues
Monocyte	Differentiates into dendritic cells and macrophages	Stored in spleen, moves through blood vessels to infected tissues
Macrophage	Phagocytosis of pathogens and cancer cells; antigen-presenting cell	Migrates from blood vessels into tissues
Natural killer cell	Kills tumour cells and virus-infected cells	Circulates in blood, migrates into tissues
Dendritic cell	Antigen-presenting cell; triggers adaptive immune response	Epithelial tissues; migrates to lymph nodes upon activation
Mast cell	Blood vessel dilation; releases heparin and histamines; recruits neutrophils and macrophages; involved in allergic reactions	Connective tissues and mucous membranes

First line of defence

Physical barriers

Physical barriers make it difficult for pathogens to adhere to cells or penetrate tissues.

Routes of pathogen entry

Pathogens can enter the body through various routes:

Mucosal surfaces:

• Airway (inhaled droplets) - e.g., influenza, meningitis
• Gastrointestinal tract (contaminated water/food) - e.g., typhoid, diarrhoea
• Reproductive tract (physical contact) - e.g., syphilis

External epithelia:

• External surface (physical contact) - e.g., athlete's foot
• Wounds and abrasions - e.g., anthrax, tetanus
• Insect bites - e.g., yellow fever, Lyme disease, malaria

Skin

The skin consists of three layers:

• Epidermis (outer layer with keratinised cells)
• Dermis (underlying layer)
• Hypodermis (subcutaneous tissue)

Protective features:

1. Keratin barrier: A waterproof protein secreted by keratinocytes that is mechanically tough and resistant to bacterial enzyme degradation

1. Stratum corneum: Upper epidermal layer of flattened dead skin cells arranged in a 'bricks and mortar' structure, forming an effective physical barrier
2. Exfoliation: Dead cells flake off, taking pathogens with them
3. Blood supply: Provides early access for white blood cells, red blood cells, and platelets to wounds

Mucous membranes

Mucous membranes are moist pink tissues lining entry points to the body, including the mouth, nasal cavity, conjunctiva, and entrances to digestive and genitourinary systems.

Protective features:

1. Cell junctions anchor epithelial cells together, restricting pathogen access
2. Cilia: Tiny hair-like structures that beat in coordinated waves to remove particles from the respiratory system (the muco-ciliary escalator)
3. Constant cell renewal: Sheets of cells continuously grow upwards to replace surface cells lost to wear or pathogen attack
4. Secretions: Mucus, lysozyme, and immunoglobulins (antibodies)

Tight junctions

Blood vessels are lined internally by endothelial cells with tight junctions that prevent pathogen entry from infected tissue into the bloodstream.

• Bacteraemia: The presence of bacteria in the bloodstream (dangerous as bacteria can travel to distant sites)
• Blood-brain barrier: A particularly important example where brain endothelial cells connected by tight junctions restrict bacterial entry to the brain

Mucus

Mucus is a slippery substance secreted by goblet cells that line mucous membranes.

Functions:

• Traps foreign substances (pathogens, dust, pollen)
• Prevents pathogen entry through cells lining the alimentary canal
• Forms cervical mucus plugs guarding the uterus during pregnancy
• Contains substances inhibiting viral replication (e.g., against rotaviruses)

Peristalsis

Peristalsis is the coordinated contraction of smooth muscle in the alimentary canal wall that moves food in one direction only.

Protective function: Prevents stasis (lack of movement) which could lead to bacterial overgrowth, as bacteria would have opportunity to proliferate.

Sphincters

A sphincter is a circular muscle maintaining constriction of a body passage, relaxing as required by normal physiological functioning.

Examples:

• Lower oesophageal sphincter (between oesophagus and stomach)
• Pyloric sphincter (between stomach and duodenum)
• Ileocaecal sphincter (between ileum and large intestine)
• Urethral sphincters (controlling urine release from bladder)
• Sphincter of Oddi (controlling digestive juice flow to duodenum)

Function: Help physically seal off compartments in the body to reduce pathogen invasion.

The microbiome: a natural barrier

The microbiome is the large and varied population of microbes living on the skin, in the intestines, colon, mouth, and vagina.

Protective mechanism: These natural microbes out-compete pathogenic ones, inhibiting their growth and multiplication.

Chemical barriers

Chemical barriers are non-specific substances secreted by epithelial tissue to prevent pathogen entry.

Urine

Urine is sterile until reaching the lower urethra. Females have shorter urethras than males and are at greater risk of urinary tract infections.

Chemical components defending against pathogens:

1. Antimicrobial peptides (AMPs): Prevent bacterial binding to epithelial cells and break down bacterial cells
2. pH: Normal human urine pH ranges from 4.5 to 8 (average 5-6, slightly acidic). Phagocytes work best in alkaline urine.

Physical defence: The flushing activity during urination (micturition) keeps pathogens away from the bladder.

Sebum and sweat

Sebum: Oily material secreted by sebaceous glands that waterproofs and lubricates skin.

Protective features:

• Skin pH around 5.5 (acidic) due to lactic acid, amino acids, and fatty acids
• Lysozyme secreted in perspiration breaks down bacterial cell walls

Saliva

Saliva is a complex mixture produced by salivary glands containing:

• Water and mucus
• Electrolytes
• Enzymes (e.g., amylase)
• Antimicrobial substances (lysozyme, immunoglobulin A (IgA), antimicrobial peptides)

Functions: Flushing action against microbes plus chemical activity through antimicrobial molecules.

Tears

Lacrimation (tear production) occurs in lacrimal glands. The tear film covers the cornea and conjunctiva, containing:

• Lysozyme, lactoferrin, lipocalin
• Antimicrobial peptides (AMPs)
• Complement proteins
• Immunoglobulin A (IgA)
• Mucins (from goblet cells)

Gastric (stomach) secretions

Parietal cells lining the stomach wall secrete hydrochloric acid, creating a highly acidic environment (pH 1-2) that discourages microbial growth and survival.

Additional protection: The enzyme pepsin (from chief cells) plays an antimicrobial role.

pH change: As food moves to the duodenum, pH rapidly changes to around 6 due to highly basic bile, providing another limiting factor for pathogen growth.

Second line of defence

Physical responses to infection

Granuloma formation

When infected cells cannot be successfully defended, they may be surrounded by a wall of dead cells forming a capsule called a granuloma. This prevents infection spreading to other areas.

• Cells inside the granuloma die, destroying the pathogens infecting them
• Debris is destroyed by surrounding macrophages
• Typical of tuberculosis and leprosy (Mycobacterium spp.)

Vomiting and diarrhoea

Vomiting (emesis): A reflex action coordinated by the brain's vomiting centre in response to pathogens in the gut (gastroenteritis). The body's way of expelling harmful substances.

• Hypersalivation occurs before vomiting to protect tooth enamel from stomach acid

Diarrhoea: Rapidly expels micro-organisms from the gastrointestinal system.

Increased urination

When the bladder lining is attacked by a pathogen, the body responds with:

• Inflammation (cystitis)
• Pollakiuria (frequent passage of small amounts of urine)

This flushing response helps remove pathogens from the urinary tract.

Wound healing

When barriers are breached, the body's priorities are:

1. Stop bleeding (haemostasis) - vasoconstriction, platelet plug formation, fibrin mesh, clot formation
2. Confront pathogens - prevent infection
3. Heal and repair - re-establish the barrier

The inflammatory response then begins.

Inflammation: a chemical response

Inflammation is a non-specific chemical response occurring at the site of infection that helps wound repair and pathogen destruction.

Five cardinal signs of inflammation

1. Dolor (pain) - due to release of chemical mediators
2. Calor (heat) - due to increased microcirculation
3. Rubor (redness) - associated with increased microcirculation
4. Tumor (swelling/oedema) - as fluids move from blood vessels to extracellular space
5. Functio laesa (loss of function) - due to pain and swelling

Mechanism of inflammation

When cells are challenged by pathogens or damaged:

1. Chemical 'alarm signals' released: Histamines (trigger vasodilation and increase vascular permeability), bradykinin, serotonin, prostaglandins (associated with pain and fever)
2. Capillaries dilate: Increased blood flow causes redness, heat, and swelling
3. Increased vascular permeability: Allows white blood cells to move from blood into tissues
4. Phagocyte recruitment: Attracted by chemotactic factors
5. Temperature increase: Endogenous pyrogens released, inhibiting pathogen growth and increasing biochemical reaction rates
6. Pathogen destruction and removal: Toxins removed, tissues repaired

Phagocytosis: a cellular response

Phagocytosis is the process by which phagocytes change shape to surround and enclose foreign particles (e.g., bacteria), then release enzymes to destroy them.

Process:

1. Phagocyte moves toward bacterium
2. Phagocyte changes shape to completely enclose bacterium
3. Lysosomes (containing destructive enzymes) approach
4. Enzymes released to destroy bacterium
5. Harmless particles released from phagocyte

Main types of phagocytes:

• Neutrophils
• Monocytes/macrophages
• Dendritic cells
• Natural killer cells

Neutrophils

Neutrophils originate in bone marrow and are the first phagocytes to move to infection sites.

Key features:

• Diapedesis: Can deform and squeeze between endothelial cells lining capillaries to move from blood to tissues
• Short-acting: Self-destruct after a few days
• Fight acute infections: Brief, severe infections
• Neutrophilia: Increased circulating neutrophils indicates active inflammation in the body

Destruction mechanism: Complex electron and ion flows create conditions in the phagocyte vacuole that kill microbes, which are then digested by enzymes (proteases).

Monocytes and macrophages

Monocytes circulate in blood until attracted to inflamed tissue, where they migrate through capillary walls and transform into:

• Macrophages ('big eaters') - long-lasting phagocytes fighting chronic infections
• Dendritic cells - antigen-presenting cells
• Osteoclasts - bone maintenance cells

Toll-like receptors (TLRs): On monocyte surfaces, recognise pathogen-associated molecular patterns (PAMPs) from bacterial cells.

The complement system

The complement system is a group of around 20 soluble proteins that assist other defence mechanisms in destroying extracellular pathogens.

Three activation pathways:

1. Classical pathway - triggered by antigen-antibody complexes
2. MB-lectin pathway - initiated by lectin binding to pathogen surfaces
3. Alternative pathway - activated directly by pathogen surfaces

Functions:

• Stimulate phagocytes to become more active
• Attract phagocytes to infection sites (recruitment of inflammatory cells)
• Opsonisation - coating pathogens to enhance phagocytosis
• Destroy pathogen membranes (killing of pathogens)

Fever: the role of pyrogens

The hypothalamus in the brain regulates body temperature like a thermostat. Normal human body temperature averages 37°C.

Fever (pyrexia): Elevation of body temperature in response to pathogens.

Pyrogens ('fire starters'): Fever-causing chemicals

Two types:

1. Exogenous pyrogens - from bacteria, viruses, endotoxins
2. Endogenous pyrogens - from phagocytic cells (interleukin I), lymphokines

Mechanism: Pyrogens act on the hypothalamus → prostaglandin derivatives → elevated temperature set-point

Purpose:

• Kill or limit pathogen growth
• Enhance white blood cell activity
• Strengthen response to pathogens

Cytokines

Cytokines are chemical messengers produced during infection that promote development and differentiation of T and B lymphocytes (third line of defence).

Examples:

• Interleukin (IL) - links innate and adaptive immune systems
• Interferons - signal viruses to stop replicating

Interferon mechanism:

• Signals uninfected cells to destroy RNA and reduce protein synthesis
• Signals infected cells to undergo apoptosis (programmed cell death)
• Activates immune cells