Biology AQA AS unit 1 new spec.

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Defence Mechanisms.

Non-specific mechanisms:

  • Do not distinguish between 1 type of pathogen and another.
  • Act immediately as either a barrier or through phagocytosis.

Specific mechanisms:

  • Distinguish between different pathogens.
  • Are less rapid but long-lasting.
  • Involve lymphocytes in 2 forms either cell-mediated involving T cells or humoral involving B cells.

The body recognises its own cells as the lymphocytes against pathogens already exist. On infection the lymphocyte that is specific is stimulated to divide. If the body did not recognise its own cells it would destroy them (auto-immune disease).

Antigenic variability - antigens of viruses are constantly changing so the only means of response is primary, e.g. flu.

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This occurs when barriers fail. Phagocytes are a type of white blood cell.
Barriers include: a protective covering that is hard to penetrate, epithelium covered in mucus that the pathogens stick to, and hydrochloric acid in the stomach to denature pathogens.


  • Large particles too big to diffuse so engulfed to form vesicles.
  • Chemical products of the pathogen act as attractants to the phagocytes.
  • Phagocytes attach themselves to the surface of the pathogen.
  • They engulf the pathogen to form a vesicle (phagosome).
  • Lysosomes fuse with the vesicle.
  • Enzymes break down the pathogen.
  • The soluble products from the breakdown of the pathogen are absorbed into the cytoplasm of the phagocyte.
  • Causes inflammation at site of infection because of release of histamine which leads to vasodilation to increase delivery of phagocytes.
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T Cells and Cell-Mediated Immunity.

Immunity - is the ability of organisms to resist infection by protecting against disease causing microorganisms. It involves the recognition of foreign material.

Antigen - is any part of the organism that is recognised as non-self by immune system.

Lymphocytes - are slower acting than phagocytes but are specific. They respond to their own cells that have been altered, not the foreign material. 2 kinds: T (cells) and B (fluids).
Cell-mediated immunity:

  • Phagocytes present digested antigens on own cell-surface membrane (antigen-presenting cells).
  • T-lymphocytes respond to the antigens on the body cell,
  • Receptors on T-helper cells fit onto antigens which activates the T cell to divide.
  • Cloned cells develop into memory cells for the future, stimulate phagocytes, stimulate B cells to divide and kill infected cells.
  • They kill infected cells by producing a protein that makes holes in the cell-surface membrane so it is freely permeable and dies.
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B Cells and Humoral Immunity.

T cells stimulate B cells to divide by mitosis so humoral is next stage of response and involves antibodies. B cells have an antibody complementary to different antigens.

Plasma cell - which secretes antibodies directly, only survive a few days and destroy pathogens and toxins. This is the PRIMARY IMMUNE RESPONSE. It is short term.

Memory cell - which lives much longer, circulate in blood (no antibodies) and divides rapidly on 2nd infection into plasma and more memory cells. New infection repulsed before harm. This is the SECONDARY IMMUNE RESPONSE. It is long term.

Humoral immunity:

  • Surface antigens of invading pathogens taken up by B cells which process and present them on their surface. T-helpers attach to these and activate division into plasma cells.
  • Plasmas produce antibodies complimentary to the antigens.
  • Antibodies attach to antigens and destroy them (this is PRIMARY).
  • Some antibodies divide into memory cells (this is SECONDARY).
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These are proteins synthesised by B cells and bind to the pathogen's antigens. There is a huge variety.

  • Y shaped: they have 4 polypeptide chains in 2 pairs.
  • 1 long pair (heavy) and 1 short (light).
  • Move as if hinged at fork of Y to help it fit round antigen.
  • Form an antigen-antibody complex as they are complementary.
  • The binding site is the variable region as is unique due to sequence of amino acids. The rest is always alike and is the constant region.

Polyclonal Antibodies- pathogens have 100s of different antigens which stimulate B cells to clone.

Monoclonal Antibodies - when 1 antibody is isolated outside body, they are used in immunoassaye.g. pregnancy test, cancer treatments as only attach to cancer cells, and transplant surgery as attack T cells that cause rejection.
Mice are exposed to non-self material, the polyclonal antibodies are extracted and mixed with cells that divide outside the body. Detergent breaks down membrane so cells fuse. Separated and cultured to form clone then extracted and humanised.

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Passive immunity - is where antibodies produced outside the body. It is short lived as body breaks down antibodies.

Active immunity - stimulates own immune system so is long lasting.

Vaccination - intro of substance that stimulates immunity without getting disease. Antigens come from weakened, dead or fragmented pathogens. Vaccinations need to be affordable, available, have few side effects, use suitable equipment and be able to be used in herd immunity.

They don't eliminate disease if: the immune system is defective,the disease develops before immunity is built, mutations arise (antigenic variability), or if the pathogen hides.

Cholera - is intestinal so inaccessible (diarrhoea), has variability and the population moves.
TB - caught if immune system compromised, mobile population and poor living conditions.
MMR - research showed cases of autism so M, M and R rose, proven to be safe now.

Scientific Evidence must be critically appraised and affirmed. Some have vested interests, other influences and can be biased.

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amazing.. thank youu!!

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