AQA Biology Unit 1 - Immunity

Defence mechanisms, phagocytosis, T-cells, B-cells, antibodies, vaccination

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Defence mechanisms

Non-specific: do not distinguish between pathogens - barrier to pathogen entry or phagocytosis

Specific: slower but longer lasting response, involve lymphocytes - cell mediated (T-cells) or humoral (B-cells) response

Lymphocytes distinguish between self and non-self cells, so our own tissues are not destroyed.

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(Barriers to entry: skin - prevents pathogens penetrating the body; mucus - pathogens stick to mucus which is transported up the trachea into the stomach; hydrochloric acid - pathogens' enzymes are denatured by low pH)

Process of phagocytosis:

1) phagocyte attracted to pathogen by the chemicals it produces along a concentration gradient

2) phagocyte binds to pathogen

3) phagocyte forms phagosome around pathogen and lysosomes move towards it

4) lysosomes release lytic enzymes into the phagosome to digest the pathogen

5) breakdown products are absorbed by the phagocyte

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Cell mediated immunity (T cells)

Respond to self-cells that have been invaded by non-self material, e.g. cancer cells, virus infected cells and phagocytes that have engulfed pathogens. They can distinguish these cells because they are antigen presenting cells, because they have antigens on their cell surface membranes.

The receptors on the T-cells are complementary to the antigens, and they fit onto antigens, which stimulates the T-cells to divide by mitosis, which develop into memory cells that will stimulate a rapid secondary response against the same pathogen in the future; stimulate phagocytes to perform phagocytosis; stimulate B cells to divide; and kill infected cells.

T-cells kill infected cells by producing a protein which makes holes in the infected cell surface membrane making it freely permeable.

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Humoral immunity (B cells)

B cells divide into two types of cells:

  • Plasma cells, secrete antibodies which destroy pathogen & toxins = primary response
  • Memory cells, live longer, circulate blood so that when they recognise the pathogen in the future, they can divide rapidly into plasma cells and quickly produce antibodies = secondary response
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