4.1. Communicable Diseases, Disease Prevention and The Immune System

Bacteria

One of the four groups of pathogens that can cause communicable diseases.

Fungi

One of the four groups of pathogens that can cause communicable diseases.

Protoctista

One of the four groups of pathogens that can cause communicable diseases.

Viruses

One of the four groups of pathogens that can cause communicable diseases.

Mycobacterium tuberculosis

Triggers inflammatory response by infecting phagocytes in lungs; infected phagocytes are sealed in waxy-coated tubercles so bacteria remain dormant.

HIV

Attachment proteins bind to complementary CD4 receptor on TH cells; HIV particles replicate inside TH cells, killing or damaging them.

AIDS

Develops when there are too few TH cells for the immune system to function, leading to inability to destroy other pathogens.

Tobacco mosaic virus

Affects plants, mainly transmitted via infected sap; contains ssRNA, which is directly transcribed by host cell to assemble new virions.

Influenza virus

Transmitted via droplet infection, contact with mucus containing virus, zoonotic infection, contact with fomites.

Malaria

Caused by the female Anopheles mosquito acting as a vector for Plasmodium spp. protoctista, which reproduces asexually in red blood cells.

Potato/tomato late blight

Caused by the protoctista Phytophthora infestans, mainly transmitted via spores.

Ring rot of potatoes

Caused by the Sepedonicus subspecies of the bacterium Clavibacter michiganensis, mainly transmitted by planting infected seeds.

Bacterial meningitis

Often caused by meningococcal bacteria A, B, C, W, X, Y, Z; affects meninges and is transmitted by droplet infection and direct contact with saliva.

Mycosphaerella fijiensis

A sac fungus that causes the leaf-spot disease black sigatoka in banana plants.

Ringworm

About 40 types of fungi cause this infection, transmitted by contact with fomites, zoonotic infection, and direct contact with infected individuals.

Athlete's foot

Caused by a range of fungi which can also affect hands or nails.

Direct transmission of pathogens

Can occur through inhalation (droplet infection).

Inhalation

Droplet infection.

Skin-to-skin contact

Exchange of fluids.

Pathogen penetration

Penetrate skin actively using enzymes or passively through wounds, hair follicles or sweat glands.

Indirect transmission of pathogens

Consumption of contaminated food & drink.

Vector transmission

Via a vector e.g. mosquitoes transmit Plasmodium parasite.

Spores

A method of indirect pathogen transmission.

Overcrowding

Increases direct transmission.

Climate influence

Determines which organisms can survive e.g. malaria is more prevalent in tropical countries, where mosquitoes (the vector) can breed.

Social factors

Influence how quickly people are treated, which can increase/decrease direct transmission.

Physical barriers in plants

Cellulose cell walls.

Physical barriers in plants

Lignified layer.

Physical barriers in plants

Waxy upper cuticle.

Physical barriers in plants

Old vascular tissue is blocked to prevent pathogens from spreading inside the plant.

Mechanical responses in plants

Guard cells close stomata.

Mechanical responses in plants

The thick polysaccharide callose is produced & deposited between the cell wall & plasma membrane to increase entry distance/limit spread.

Necrosis

Injury activates intracellular enzymes in plants that kill cells near the site of infection to prevent pathogen from spreading.

Canker

Necrosis of woody tissue.

Chemical defences in plants

Terpenoids (essential oils) e.g. menthols act as antibacterials.

Chemical defences in plants

Phenols e.g. tannin inhibit insects from attacking by interfering with digestion.

Chemical defences in plants

Alkaloids e.g. caffeine & morphins deter herbivores from feeding because they taste bitter.

Chemical defences in plants

Defensins (cysteine-rich proteins) inhibit transport channels.

Chemical defences in plants

Hydrolytic enzymes e.g. chitinases break down cell wall of invading organisms.

Barriers to infection in animals

Skin is tough keratin layer.

Barriers to infection in animals

Blood clotting prevents pathogens from entering through skin lesions.

Barriers to infection in animals

Hydrochloric acid in stomach kills bacteria.

Barriers to infection in animals

Harmless bacteria in gut & on skin surface increase interspecific competition with pathogens.

Barriers to infection in animals

Mucous membranes trap pathogens and may secrete antimicrobial enzymes.

Expulsive reflexes

Body attempts to force foreign substances out: Irritation of mucous membranes in nostrils causes sneezing.

Expulsive reflexes

Irritation of ciliated epithelium in respiratory tract causes coughing.

Nonspecific immune response

Inflammation.

Nonspecific immune response

Phagocytosis.

Nonspecific immune response

Digestive action of lysozymes.

Nonspecific immune response

Production of interferon (antiviral agent).

Process of inflammation

1. Damaged vessels release histamines, causing vasodilation. 2. Blood flow & permeability of blood vessels increase. 3. White blood cells & plasma move into the infected tissue.

Blood clotting process

1. Blood platelets form plug & release chemicals that enhance clotting e.g. thromboplastin. 2. Prothrombin changes into thrombin, its active form. 3. Fibrinogen changes into insoluble fibrin which covers wound.

White blood cells in phagocytosis

Types of white blood cell involved in phagocytosis.

Neutrophils

A type of white blood cell important in the immune response.

Macrophages

Cells that can become antigen-presenting cells.

Phagocytosis

The process by which phagocytes destroy pathogens.

Phagosome

A vesicle formed around a pathogen during phagocytosis.

Phagolysosome

The fusion of a phagosome with a lysosome.

Lysozymes

Digestive enzymes found in lysosomes that damage bacterial cell walls.

Antigen-Presenting Cells (APCs)

Cells that display antigens from pathogens on their surface to enhance recognition by T helper cells.

Cytokines

Signaling molecules secreted by APCs that stimulate specific immune responses.

Leishman stain

A stain used in preparing blood samples for microscopic observation.

Cell-Mediated Immune Response

An immune response involving T lymphocytes that bind to foreign antigens.

Humoral Immune Response

An immune response involving B lymphocytes that produce antibodies.

TH Lymphocytes

T helper cells that bind to foreign antigens and stimulate immune responses.

Cytotoxic T Cells (TC)

T cells that secrete perforin to destroy infected cells.

Plasma Cells

B cells that secrete antibodies.

Antibody

Proteins secreted by plasma cells with a specific structure to bind antigens.

Memory Cells

Specialized TH/B cells that remain in low levels in the blood and can rapidly divide upon re-encountering a pathogen.

Primary Immune Response

The initial response of the immune system to a pathogen.

Secondary Immune Response

The immune response upon re-exposure to the same pathogen, typically faster and stronger.

Agglutinins

Antibodies that form antigen-antibody complexes to enhance phagocytosis.

Opsonins

Molecules that mark microbes for phagocytosis.

Antitoxins

Antibodies that neutralize toxins by making them insoluble.

Interleukins

Cytokines that stimulate clonal expansion of T and B cells.

Complement

A system of proteins that enhances the ability of antibodies to clear pathogens.

T Helper Cells

A type of T cell that secretes cytokines and helps activate other immune cells.

T Killer Cells

T cells that destroy infected cells.

T Regulator Cells

T cells that suppress other immune cells to prevent autoimmune disease.

Variable Region

The part of an antibody that has a specific tertiary structure complementary to an antigen.

Constant Region

The part of an antibody that remains the same regardless of the antigen.

Primary immune response

Initial response of the immune system upon first exposure to a pathogen.

Secondary immune response

Faster rate of antibody production, shorter time lag between exposure and antibody production, higher concentration of antibodies, antibody level remains higher after the secondary response, and pathogen usually destroyed before any symptoms.

Passive immunity

Involves antibodies from an external source, no memory cells are produced, and it provides short-term immunity.

Active immunity

Involves lymphocytes producing antibodies after direct contact with an antigen, resulting in long-term immunity due to memory cells.

Passive natural immunity

Antibodies obtained through breast milk or across the placenta.

Passive artificial immunity

Antibodies obtained through anti-venom or needle stick injections.

Active natural immunity

Humoral response generated from an infection.

Active artificial immunity

Immunity developed through vaccination.

Autoimmune disease

Condition where the immune system produces antibodies against its own tissues.

Rheumatoid arthritis

An autoimmune disease where the immune system targets synovium lined joints, causing inflammation.

Lupus

An autoimmune disease that results in inflammation throughout the body.

Vaccination principles

1. Vaccine contains dead/inactive form of a pathogen or antigen. 2. Triggers primary immune response. 3. Memory cells are produced and remain in the bloodstream for a rapid secondary response. 4. Pathogen is destroyed before it causes symptoms.

Endemic

A disease that occurs routinely in a geographical area.

Epidemic

A temporary rapid increase in the incidence of a disease in a geographical area.

Role of vaccines in epidemics

Routine vaccination of 80-90% of the population reduces available carriers of pathogens, resulting in herd immunity.

Natural sources of medicines

Microorganisms (e.g., streptomycin, neomycin), fungi (e.g., penicillin), and plants (e.g., Taxol from yew, quinine from cinchona, digoxin from foxglove).

Personalised medicine

A field that uses genome sequencing to predict an individual's response to diseases or certain medicines for targeted prescriptions.

Synthetic biology

Engineering that targets biochemical processes.

Benefits of antibiotics

Effectively reduce the population of bacterial colonies and prevent protein synthesis or inhibit nucleic acid formation.