What are the primary defence systems?
skin -
- outermost layer provides a physical barrier and is overed by a
- tough layer of dead cells containing large amounts of the protein keratin.
Sebaceous glands are associated with hair follicles.
- They secrete a chemical called sebum, which maintains skin moisture and slightly lowers skin pH.
- Low pH (acidity) inhibits the growth of bacteria and fungi.
Mucous membranes
- thinner skin found in the reproductive parts and the airway leading to the lungs.
- Mucus is a type of physical barrier: pathogens and harmful
particles become trapped in it and are then swallowed or expelled.
- It also has antiseptic properties because of the presence of the anti-bacterial enzyme lysozyme
How does blood clot
- Initiated by platelets
- platelets aggregate at the
site forming a temporary plug - They then release clotting factors that trigger the clotting process
- clotting from platelets produced enzyme called thrombonin
- Thrombin converts the protein fibrinogen into insolubale fibrin
- It forms a mesh in the cuts, trapping more platelets and blood cells
Differences between the innate
immune system and the adaptive
immune system
- The innate immune system responds to broad categories of pathogen. It does not change during an organism’s life eg: phagocytes
- Adaptive immune system responds in a specific way to particular pathogens. builds up memory of pathogens, so
it offers more effective protection against diseases eg: lymphocytes
Infection control by phagocytes
- white blood cells - next line of defence
- Phagocytes squeeze out through pores in the walls of capillaries and move to sites of infection.
- They engulf pathogens by endocytosis and digest them using enzymes from lysosomes
How do Lymphocytes work
- Lymphocytes produce antibodies
- Antibodies have two functional parts: a hypervariable region that
recognizes and binds to a specific molecule on a pathogen and another region that helps the body to fight the pathogen - Lymphotcytes multiply when there’s a pathogen
How do antigens act as recognition molecules
- The molecules used for recognition are called antigens and are located on the surface of the pathogen
- The immune response is the production of specific antibodies
- lymphocytes produce antibodies that bind to the antigen
- Protrusions on antigens match hollows on the corresponding antibody & positive charges match negative charges, this binding is irreversible
How do T cells help the B cells
- Pathogens are ingested by macrophages(phagocytic white blood cell).
- Antigens from the pathogens are then displayed in the plasma membrane of the macrophage.
- Cells called helper T-lymphocytes each have an antibody like receptor protein in their plasma membranes. - This can bind to antigens displayed by macrophages.
- T-cells bind with and are activated by the macrophage
- Activated T-cells then bind to lymphocytes B-cells. Only B-cells with a specific receptor protein to which the antigen binds are selected and undergo the binding process.
- The T-cell activates the selected B-cells, both by means of the binding and by release of a signalling protein
What happens to the activated B - cell
- B-lymphocytes become
activated but don’t immediately produce antibodies because there are too few many of them - Instead the activated B- cells
divide repeatedly by mitosis to form a clone of cells that produce the same antibody - B-lymphocytes grow in size and develop an extensive ER with many ribosomes attached to it, along with a large Golgi apparatus.
- So, there’s rapid production of antibodies by protein synthesis and are called plasma B-cells
How are memory cells created
- The plasma cells that secrete
antibodies lost after an infection because the antigens associated with that infection are not present - A smaller number of cells
in the clone don’t immediately secrete antibodies but remain
for a long time after the infection - These memory cells remain inactive until that pathogen re-enters the body
Transmition of HIV
- viruses in blood, semen, vaginal fluids, rectal secretions and breast milk
- sex without a condom, during which abrasions to the mucous membranes can cause minor bleeding
- sharing of hypodermic needles by intravenous drug users
- transfusion of infected blood, or blood products such as Factor VIII
- childbirth and breastfeeding
- HIV is a retrovirus so has genes made of RNA. It uses reverse transcriptase to produce DNA copies of its genes after entering a host cell. Antiretroviral drugs
inhibit reverse transcriptase
How do vaccines work
- vaccines contain either antigens that allow a pathogen to be recognized by the immune system or nucleic acids from which antigens can be made
- stimulate a primary immune response, by activation of T-cells and B-cells & production of plasma cells and then specific antibodies
- If a vaccine successfully triggers such immunity, the pathogenic
microorganism will be destroyed by a secondary immune response if it ever enters the body
Formula for the % of people that have to be immune for the pop. to be protected
(1−1/R ) × 100%
