communicable diseases

• Communicable diseases → diseases that can spread between organisms
• Bacterial diseases

  • Tuberculosis (TB) → affects animals including humans

  → spread via infected droplets

  • Bacterial meningitis → affects humans

  • Ring rot → affects potatoes and tomatoes

  → spread through direct contact
• Protoctist diseases

  • Malaria → affects animals including humans

  → spread by a vector (mosquitos)

  • Potato (or tomato) late blight

  → affects potatoes (or tomatoes)

  → produces spores during reproduction that are spread in the air to other hosts
• Viral diseases

  • HIV/AIDS → affects humans

  → spread through sexual contact

  • Influenza → affects animals including humans

  → spread through infected droplets

  • Tobacco mosaic virus → affects plants

  → spread by direct contact
• Fungal diseases

  • Black sigatoka → affects bananas

  → spread via spores

  • Ringworm → affects cattle

  • Athlete’s foot → affects humans

  → spread by direct and indirect
• Transmission

  1. Direct transmission

  2. Indirect transmission
• Direct transmission

  • Through infected droplets (not plants)
  • Sexual contact
  • Touch
• Climate

  Affects transmission e.g. having the ideal temperature for a vector to survive and reproduce
• Living conditions


  Affect transmission e.g. overcrowding makes transmission easier, poor hygiene practises
• Social factors


  Affect transmission e.g. lack of sexual education and hygiene education, lack of good healthcare facilities and poor drug or vaccine availability
• Measures to decrease transmission → mask wearing and improving ventilation

  → meeting less people

  → destroying vectors

  → quarantine of infected individuals

  → increasing hygiene practises e.g. hand washing

  → restrict travel
• A vector is an organism which carries a pathogen without getting symptoms of the disease
• Plant defences against pathogens

  • Cell walls act as a physical barrier to pathogens → callose can be deposited between the cell wall and plasma membrane which acts as an extra barrier

  • Waxy cuticles are a physical barrier

  • Production of toxic antimicrobial chemicals

  → e.g. production of chitinases to break down chitin cell walls of fungi

  → e.g. phytoalexins inhibit growth and reproduction of bacteria and fungi

  • Leaf drop → some plants will lose leaves to prevent pathogens from spreading around the plant
• Skin

  • Physical barrier
  • Produces antimicrobial chemicals which lower the pH
• Blood clotting

  1. Blood clots are a mesh of protein fibres, platelets, and erythrocytes

  2. Platelets release a substance which results in a soluble plasma protein (fibrinogen) being converted into an insoluble protein (fibrin) which can trap erythrocytes and platelets to form a clot

  3. Blood clots are a physical barrier at wounds
• Wound repair

  1. Skin cell division

  2. Collagen fibres
• Inflammation

  1. Mast cells (a type of white blood cell) release histamine

  2. Histamine increases blood flow through vasodilation, and increases permeability of the capillaries

  3. More white blood cells can enter the affected area
• Expulsive reflexes

  Coughing and sneezing expel pathogens from the respiratory tract
• Mucous membranes


  • Found at openings to the body e.g. the trachea
  • Can contain goblet cells which secrete mucus
  • Mucus traps pathogens and contains lysozyme
• Antigens

  • Molecules found on a cell surface which identify the cell → specific to each type of cell and organism

  • Foreign antigens are identified by the immune system → can be on pathogens, abnormal cells (e.g.cancer cells), cells from other organisms (e.g. transplants), or toxins

  • Often proteins
• • Opsonins → antibodies which bind to antigens of pathogens to increase recognition by phagocytes

  • Agglutinins → antibodies which clump pathogens together so that many can be engulfed at once

  • Cytokines → molecules released by T helper cells which attract phagocytes
• Phagocytosis

  • Phagocytes are a type of white blood cell

  • Neutrophils and monocytes are types of phagocyte

  • Phagocytosis destroys pathogens

  → pathogens produce chemicals which attract phagocytes

  → phagocytes recognise the foreign antigens on pathogens

  → phagocytes engulf pathogens

  → the pathogen is contained in a phagosome

  → the phagosome fuses with a lysosome to form a phagolysosome

  → the pathogen is digested by lysozymes from the lysosome

  → antigens from the pathogen are presented on the plasma membrane of the phagocyte
• Observing blood smears

  • Neutrophils → have a multi-lobed nucleus

  → can have granules in the cytoplasm

  • Erythrocytes (red blood cells) → no nucleus

  • Lymphocytes → large nucleus which takes up most

  of the cell volume

  • Monocytes → have a kidney bean shaped nucleus

  → the largest type of white blood cell

  • Platelets → small fragments of cells
• T lymphocytes

  • T lymphocytes have receptors on their surface which are

  complementary to one specific antigen → bind to antigens

  presented by antigen-presenting cells e.g. phagocytes

  • Binding of an antigen to receptors on a T helper cell activates the

  specific T helper cell → this is called clonal selection

  • The T helper cell divides by mitosis to produce clones of itself

  this is called clonal expansion

  • Some clones differentiate into T killer cells, T memory cells and T regulatory cells

  • Some clones remain as T helper cells
• T killer cells → kill infected cells by producing chemicals that result in cell lysis

  • T helper cells → release cytokines (including interleukins) which stimulate clonal expansion in B lymphocytes and attract phagocytes

  • T memory cells → remain in the blood for future immunity

  • T regulatory cells → suppress immune system cells to help prevent autoimmunity
• B lymphocytes

  • B lymphocytes have antibodies on their surface which are complementary to one specific antigen

  • Binding activates the specific B lymphocyte

  → this is called clonal selection

  → interleukins from T helper cells are also required

  • The B lymphocyte divides by mitosis and produces clones of itself

  → this is called clonal expansion

  • The clones differentiate into plasma cells and B memory cells
• Plasma cells → produce and release antibodies complementary to the antigen

  • B memory cells → remain in the blood for future immunity
• Antibodies

  • Proteins consisting of four polypeptide chains

  • The variable regions bind to a specific complementary antigen

  → the tertiary structure of these regions is very important

  • Constant regions bind to plasma membranes of immune system cells

  • Hinge regions give flexibility

  • Disulfide bridges between the heavy chains and light chains form the quaternary structure
• • Antibodies called agglutinins cause agglutination of pathogens

  → an antibody has two antigen binding sites

  → pathogens are clumped together and cannot reproduce

  → clumps of pathogens are engulfed by the phagocytes

  • Anti-toxins can bind to toxins to inactivate them and cause

  them to be engulfed by phagocytes

  • Prevent pathogens binding to host cells
• Primary and secondary response

  • Primary response (the immune response to a new foreign antigen)

  → the antigen is presented on antigen-presenting cells

  → clonal selection and clonal expansion of T and B lymphocytes

  → antibodies are produced by plasma cells

  → T and B memory cells are produced

  • Secondary response (the immune response to a known foreign antigen)

  → receptors and antibodies on T and B memory cells bind to the complementary antigen

  → clonal selection and clonal expansion happen more quickly to produce plasma cells

  → plasma cells rapidly produce antibodies
• Types of immunity

  • Natural active immunity → primary response

  triggered through infection with a pathogen

  • Artificial active immunity → primary response

  triggered through vaccination

  • Natural passive immunity → a baby receiving

  antibodies from the mother through the

  placenta or breastmilk

  • Artificial passive immunity → injecting

  somebody with antibodies

  • Active immunity means that lymphocytes are

  activated and memory cells are produce
• Autoimmune disease

  A disease in which the antigens on an individual's own cells are recognised as non-self (foreign) by the immune system, resulting in an abnormal immune response
• Autoimmune disease

  1. Antibodies against self-cells are produced

  2. Phagocytes and T killer cells act on self-cells
• Autoimmune disease

  • Can have genetic or environmental causes
• Autoimmune diseases

  • Lupus
  • Rheumatoid arthritis
• Lupus


  • Patients have antibodies against the cell-surface antigens of connective tissue
  • Causes a butterfly-shaped rash on cheeks, pain in joints, inflammation, fatigue, fever
• Rheumatoid arthritis

  • Patients have antibodies against the cell-surface antigens of tissue surrounding the joints
  • Causes joint pain and inflammation
• Treatment

  Often treated with immunosuppressant drugs
• Vaccines

  Contain a safe form of an antigen, could be an inactive form of a pathogen
• Vaccination

  1. Injecting the foreign antigen into the blood

  2. Triggers the primary immune response

  3. Antibodies and memory cells are produced