C3.2

Pathogen

A disease-causing organism.

Types of human pathogens

Viruses, bacteria, fungi, and protists.

Archaea and disease

Archaea are not known to cause disease in humans.

Infectious disease

A disease caused by a pathogen that can be transmitted between hosts.

Importance of observation in disease control

Careful observation has led to major advances in controlling infectious disease.

Childbed fever investigation

Observation showed infections after childbirth could be reduced by handwashing.

Cholera investigation

Observation linked cholera outbreaks to contaminated drinking water.

Skin as a primary defence

The skin prevents pathogen entry through physical and chemical barriers.

Physical barrier of skin

Tightly packed cells prevent pathogen penetration.

Chemical barrier of skin

Sebum lowers skin pH and inhibits bacterial growth.

Mucous membranes

Line body openings and protect against pathogens.

Mucus secretion

Traps pathogens before they enter tissues.

Cilia in airways

Move mucus toward the throat to be swallowed or expelled.

Lysozyme

Enzyme that breaks down bacterial cell walls.

Role of blood clotting in defence

Prevents pathogen entry through damaged skin.

Platelets

Cell fragments involved in blood clotting.

Release of clotting factors

Platelets release factors when blood vessels are damaged.

Clotting cascade

A chain reaction of enzyme activations.

Thrombin

Enzyme produced during clotting.

Fibrinogen

Soluble plasma protein involved in clot formation.

Fibrin

Insoluble protein that forms fibres.

Blood clot formation

Fibrin mesh traps erythrocytes and platelets to seal wounds.

Innate immune system

A non-specific defence mechanism present from birth.

Specificity of innate immunity

Responds to broad categories of pathogens.

Adaptability of innate immunity

Does not change during an individual's lifetime.

Cells of innate immunity

Includes phagocytes.

Adaptive immune system

A defence system that responds specifically to particular pathogens.

Specificity of adaptive immunity

Targets specific antigens.

Memory in adaptive immunity

Becomes more effective after repeated exposure.

Cells of adaptive immunity

Lymphocytes.

Phagocytes

Cells that engulf and digest pathogens.

Amoeboid movement

Movement of phagocytes through tissues to infection sites.

Recognition of pathogens

Phagocytes recognize non-self molecules.

Phagocytosis

Endocytosis of pathogens by phagocytes.

Role of lysosomes

Contain enzymes that digest pathogens.

Lymphocytes

Cells responsible for specific immune responses.

Locations of lymphocytes

Circulate in blood and are found in lymph nodes.

Types of lymphocytes

B-lymphocytes and T-lymphocytes.

B-lymphocytes

Lymphocytes that produce antibodies.

Antibody specificity of B-cells

Each B-cell produces one specific antibody.

Antibody diversity

Large variety of B-cells exist, each specific to a different antigen.

Antigen

A molecule that triggers an immune response.

Nature of antigens

Usually proteins or glycoproteins.

Location of antigens

Found on the surface of pathogens.

Red blood cell antigens

Can trigger immune responses in incompatible transfusions.

Helper T-lymphocytes

T-lymphocytes that activate B-cells.

Antigen specificity of helper T-cells

Each helper T-cell responds to one specific antigen.

Activation of helper T-cells

Occurs when they bind to their specific antigen.

Activation of B-lymphocytes

B-cells must be activated before producing antibodies.

Requirements for B-cell activation

Binding to antigen and contact with an activated helper T-cell.

Direct cell contact in activation

B-cell and helper T-cell must physically interact.

Role of signalling molecules

Helper T-cells release cytokines that activate B-cells.

Clonal expansion

Activated B-cells divide by mitosis.

Purpose of clonal expansion

Produces large numbers of cells to fight infection.

Plasma cells

B-cells specialized for antibody secretion.

Antibody secretion

Plasma cells release large quantities of antibodies.

Memory B-cells

Long-lived B-cells retained after infection.

Primary immune response

First response to an antigen.

Characteristics of primary response

Slow and produces fewer antibodies.

Secondary immune response

Response upon re-exposure to the same antigen.

Characteristics of secondary response

Faster, stronger, and produces more antibodies.

Immunity

Ability to eliminate an infectious disease.

Basis of immunity

Retention of memory cells.

Role of memory cells

Rapid antibody production during secondary exposure.

HIV

Human immunodeficiency virus.

Transmission of HIV

Occurs via exchange of body fluids.

Examples of HIV transmission

Sexual intercourse, blood transfusion, shared needles, mother to child.

Target cells of HIV

Helper T-lymphocytes.

Effect of HIV infection

Destroys helper T-cells.

Impact on immune system

Reduces antibody production.

AIDS

Condition resulting from severe immune system damage.

Antibiotics

Chemicals used to treat bacterial infections.

Mode of action of antibiotics

Block bacterial metabolic processes.

Selectivity of antibiotics

Target bacteria but not eukaryotic cells.

Why antibiotics do not affect viruses

Viruses lack their own metabolism.

Antibiotic resistance

Ability of bacteria to survive antibiotic treatment.

Origin of resistance

Random genetic mutations.

Natural selection in resistance

Resistant bacteria survive and reproduce.

Multiresistant bacteria

Bacteria resistant to multiple antibiotics.

Importance of responsible antibiotic use

Slows development of resistance.

Zoonoses

Diseases transmitted from animals to humans.

Examples of zoonotic diseases

Tuberculosis, rabies, Japanese encephalitis, COVID-19.

Significance of zoonoses

Major source of emerging infectious diseases.

Vaccines

Preparations that stimulate immunity without causing disease.

Contents of vaccines

Antigens or DNA/RNA coding for antigens.

Effect of vaccination

Induces primary immune response.

Immunization

Process of developing immunity through vaccination.

Herd immunity

Protection of a population when many individuals are immune.

Mechanism of herd immunity

Transmission chains are disrupted.

Benefit of herd immunity

Protects individuals who cannot be vaccinated.

Role in epidemic prevention

Reduces spread of infectious diseases.

Percentage difference

Compares two values at the same time.

Formula for percentage difference

(difference ÷ average) × 100.

Percentage change

Compares a value over time.

Formula for percentage change

(change ÷ original value) × 100.