2.4 (1) disease and defence mechanisms

what is a disease?

what is a disease?

a physical or mental disorder or malfunction with a characteristic set of symptoms.

this excludes disorders resulting from physical traumas.

what are communicable diseases?

diseases that can be spread by pathogens between organisms, either of the same species or sometimes between different species

what is a pathogen?

a microorganism or virus that enters another organism and causes communicable diseases

what is the host?

the organism which a pathogen infects

what are the four main categories of pathogens?

• bacteria

• viruses

• fungi

• protista

what is an antigen?

a foreign structure on the surface of a pathogen that can be detected by antibodies and trigger an immune response.

they can be glycoproteins, glycolipids or polysaccharides

what is an antibody?

a protein released by B lymphocytes that detects and binds to antigen, which helps neutralise or eliminate the attached pathogen.

how does the skin defend us against infection?

• it is a physical barrier consisting of keratin (hair) that prevents microorganisms from entering the body.

• dead cells on the outer layer are difficult for pathogens to penetrate.

• produces sebum (oil) which kills bacteria.

• sebaceous glands on skin secrete lactic acid and fatty acids to prevent the growth of most bacteria.

what are mucous membranes?

how do mucous membranes defend us from infection?

• soft areas on the skin that are kept moist by mucous, such as the nose, eyes, airways, anus, genital openings.

• the barrier they form is weaker than skin.

• they secrete lysozymes, which kill bacteria by damaging their cell walls.

• tears and mucus contain lysozymes.

how do cilia and mucus defend us from infection?

• cilia and mucus in the nose trap pathogens before they enter the respiratory system.

• cilia in trachea waft mucus containing pathogens towards the throat where it is swallowed in the stomach.

how does the stomach defend us from infection?

• contains hydrochloric acid which destroys pathogens before they travel further down the digestive system.

what is phagocytosis?

the process where phagocytes engulf and destroy material

explain the process of phagocytosis.

• phagocyte is attracted to the foreign antigens and the chemical products of the pathogen.

• phagocyte moves towards pathogen via chemotaxis.

• the cytoplasm of the phagocyte moves around the pathogen, engulfing it via endocytosis.

• the pathogen is now enclosed in a phagocytic vacuole in the cytoplasm of the phagocyte.

• lysosomes fuse with the phagocytic vacuole.

• lysozymes from lysosomes digest the pathogen by hydrolysing the bacterium.

• phagocyte absorbs the products from pathogen hydrolysis.

• there is breakdown debris of the pathogen.

• phagocyte then becomes the pathogen’s antigen-presenting cell, which activates an immune response if the antigen is recognised as foreign.

what are T-lymphocytes?

explain the 1st stage of the cell-mediated response.

• a type of WBC that has receptor proteins on its surface which bind to complementary antigens presented to it by phagocytes.

• this activates the T-cell.

• cytokines are released that stimulate rapid mitosis/clonal expansion of:

  • helper T-cells

  • cytotoxic T-cells (aka T-killer cells)

explain the difference responses of different T-cells.

what is the 2nd stage of the cell-mediated response?

• helper T-cells (TH cells) release chemical signals that stimulate phagocytes, B-cells and cytotoxic T-cells (T_c cells) to divide.

• B-cells (aka B effector or plasma cells) secrete antibodies.

• cytotoxic T-cells (T_c) secrete perforin enzymes to kill abnormal foreign cells/infected cells.

• B-cells and helper T-cells also become memory cells or trigger humoral response.

what are memory cells?

• specialised helper T-cells and B-cells produced from primary immune response.

• remain in low levels in the blood after the first infection.

• divide rapidly by mitosis if the organism encounters the same pathogen again (secondary immune response).

how does the secondary immune response differ from the primary immune response?

in the secondary response:

• memory cells rapidly divide by mitosis.

• shorter time lag between exposure to the pathogen and antibody production.

• faster rate of antibody production.

• higher concentration of antibodies in the bloodstream.

• antibodies persist in the bloodstream for longer/remain high for longer after the second immune response.

• pathogen is usually destroyed before any symptoms are shown.