6. Immunity.

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Defence Mechanisms

IMMUNITY - having overwhelmed a pathogen for the first time, the body's defences can be better prepared for a second infection and can repel it before it can cause any harm.

There are 2 main types:

Non - Specific: don't distinguish between pathogens, but respond to them all in the same way. They act immediately. They take 2 forms: a barrier to the entry of pathogens/phagocytosis.

Specific: distinguish between pathogens. Their responses are less rapid but are long lasting. It involves a white blood cell called a LYMPHOCYTE. There are 2 types: cell mediated responses and humoral responses.

Lymphocytes: theres a high probabilty, when a pathogen enters the body, one of the lymphocytes will have a protein on its surface thats complementary to one of the proteins on the pathogens surface. When an infection occurs, the one type already present that has the complementary proteins is stimulated to build up its numbers to a level where it can be effective in destroying it.

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1. Chemicals from the pathogen ATTRACT phagocytes - they move towards it.

2. Phagocytes ATTACH themselves to the pathogen.

3. They ENGULF the pathogen to form a VESICLE (phagosome).

4. Lysosomes move towards the vesicle and FUSE with it.

5. Enzymes in the lysosome DIGEST the pathogen.

6. Soluable products are then ABSORBED into the cytoplasm of the phagocyte.

Barriers to entry:

  • SKIN
  • Epithelia covered in mucus - pathogens stick to mucus, CILIA sweep it to stomach
  • Hydrochloric acid in the stomach - low pH so the enzymes of most pathogens are killed.
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Cell Mediated Immunity


ANTIGEN - It is a protein that stimulates an immune response.

The presence of an antigen triggers the production of an antibody. 

Lymphocytes - the specific immune response depends on a type of white blood cell called a lymphocyte. There are 2 types of lymphocyte:

B Lymphocytes: associated with humoral immunity (mature in bone marrow).

T Lymphocytes: associated with cell mediated immunity (mature in the thymus gland).

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Cell Mediated Immunity - T Lymphocytes

T cells respond to an organisms own cells that have been invaded by non - self material (and transplanted material).

They can distinguish these invader cells because:

  • Phagocytes that have engulfed and digested a pathogen, present some of the pathogen's antigens on their own cell surface membrane.
  • Body cells invaded by a pathogen manage to present some of the pathogens antigens on their cell surface membranes.

These cells are called ANTIGEN-PRESENTING CELLS - present antigens of other cells on their cell surface membrane.

T cells only respond the antigens that are attatched to a body cell - cell mediated immunity.

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T Cell Response

The response of T cells to infection by a pathogen:

1. Pathogens invade bodycells/taken in by phagocytes.

2. The phagocyte places antigens from the pathogen on its cell surface membrane.

3. Receptors on certain T Helper cells fit exactly onto these antigens.

4. Activates other T cells to rapidly divide by mitosis to form a clone.

5. The cloned T cells:

        • Develop into memory cells that enable a rapid response in the future.
        • Stimulates phagocytosis.
        • Stimulate B cells to divide.
        • Kills infected cells.
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How T cells kill infected cells

  • They produce a PROTEIN that makes holes in the cell surface membrane.
  • This hole means the cell becomes freely permeable to all substances and dies as a result.
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B cells and Humoral Immunity

  • It involves ANTIBODIES (soluable in blood and tissue fluid).
  • Each type of B cell produces a different antibody that responds to one specific antigen.
  • When the antigen enters the blood/tissue fluid, there will be one type of B cell that has an antibody on its surface whose shape exactly fits the antigen.
  • The antibody therefore attaches onto this complementaty antigen.
  • This type of B cells divides by MITOSIS to form a clone of identical B cells, all of which produce an antibody thats specific to the foreign antigen.

Many B cells make clones, each produces its own type of antibody. Develop into 2 types:

1. PLASMA CELLS - secrete antibodies directly. They only survive for a few days. These antibodies destroy the pathogen and its toxins - responsible for the IMMIDIATE DEFENCE - primary immune response.

2. MEMORY CELLS - circulate in the blood/tissue fluid. When then encounter the same pathogen at a later date, the immune response is quicker, they rapidly divide into plasma cells and more memory cells. Provides long-term immunity - secondary immune response. The pathogen is destoyed before it can cause harm.

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Role of B cells

1. The surface antigens of the invading pathogen are taken up by B cells.

2. The B cells process the antigens, and present them on their surface.

3. T helper cells attach to the processed antigens - and activate them.

4. The B cells are activated to divide by MITOSIS to give a clone of plasma cells.

5. The cloned plasma cells produce antibodies that exactly fit the antigens on the pathogens surface.

6. The antibodies attach to antigens on the pathogen and destroy them.

7. Some B cells develop into memory cells - respond to future infections by dividing rapidly and developing into plasma cells that directly secrete antibodies.

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Antigenic Variability and Antibodies

ANTIGENIC VARIABILITY - the antigens are constantly changing.

  • Subsequent infections are therefore likely to be caused by different varities of the pathogen - their antigens will not correspond to the antibodies/memory cells formed during previous infections.
  • With no memory cells to stimulate antibody production, the only means of overcoming the pathogen is the primary response.


  • Made up of 4 polypeptide chains. The chains of one pair are long = heavy chains. And the chains of the other pait are shorter = light chains.
  • To help the antibody fit around the antigen, the can change chape by moving.
  • They have a binding site that fits precisly onto the antigen to form an antigen-antibody complex. The binding site is different on each antibody - VARIABLE REGION.
  • Each site consists of a sequence of amino acids that forms a 3D shape that binds directly onto a single type of antigen.
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Monoclonal Antibodies

    •  Antibodies that bind to ONE specific antigen.

They have a number of useful functions:

  • Separation of a chemical from a mixture.
  • Immunoassy - method of calculating how much substance is in a mixture.
  • Cancer treatment - monoclonal antibodies can be made so they ONLY attach to cancer cells - can be used to activate a CYTOTOXIC drug (one that kills cells). The drug will only only be activated by cells to which the monoclonal antibody is attached. The cancer cells will then be destroyed.
  • Transplant Surgery - a transplanted organ will always suffer some sort of rejection because of the action of the T cells. Monoclonal antibodies can 'knock-out' these specific T cells.
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VACCINATION - introduction of a substance into the body with the intention of stimulating active immunity against a particular disease.

Features of a successful vaccination programme:

  • A suitable vaccine must be economically available, in sufficient quantites to immunise all of the vunerable population.
  • There must be few side affects.
  • Means of producing, storing and transporting the vaccine must be available.
  • Must be a means of administering the vaccine properly, at the right time. Involves training staff with appropriate skills.
  • It must be possible to vaccinate the vast majority - herd immunity.
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