2.4 (1) disease and defence mechanisms

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.