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Defence mechansims

Non specific- mechanisms that do not distingish between one type of pathogen and another, but respond to all of them in the same way. These mechanisms act immediatley and take two forms:

  • A barrier to the entry pathogens
  • Phagocytosis

Specific- mechansims that do distingish between different pathogens. The responses are less rapid but provide long-lasting immunity. The responses involve a type  of white blood cell called a lympocyte and again take two forms:

  • Cell-mediated responses invloving T lymphocytes
  • Humoral response involving B lymphocytes
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How lymphocytes recognise their own cells

There are about 10 million different lymphocytes each will have a protein on its surface that is complementary to one of the proteins of the pathogen.

  • In the fetus, these lymphocytes are constantly colliding with other cells.
  • Infection in the fetus is rare.
  • Lymphocytes will therefore collide with the bodys own material.
  • Some of the lymphocytes will have receptors that exactly fit those of the body's own cells. These lymphocytes either die or as suppressed.
  • The only remaining lymphocytes are those that fit foreign material and therefore only respond to foreign material.
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Summary of defence mechanisms

Defence mechanisms

                 Non Specific                                            Specific

Reponse is immediate and the same for       Response is slower and specific to each pathogen             all pathogens  

Phsyical barrier  Phagocytosis     Cell-mediated response  Humoral response

E.g. Skin                                                                                T lympocytes                                B lymphocytes

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Barriers to entry

A protective covering- The skin covers the body surface, providing a physical barrier that most pathogens find hard to penetrate

Epithelia covered in mucas- Acts as a further defence for example in the lungs pathogens stick to mucus which is transported away by cilia up the trachea to be swallowed into the stomach

Hydrocholoric acid in the stomach- Provides a low pH that enzymes of most pathogens are denatured and therefore the organisms are killed

What is phagocytosis?

Large particles have to be engulfed by cells- this is called phagocytosis. In the blood, the types of white blood cells that carry out phagocytosis are known as phagocytes. They provide an important defence against the pathogens.

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How phagocytosis works:

  • The pathogen is recognsied as foreign
  • Chemical products of the pathogen acts as attractants, casuing phagocytes to move towards the pathogen.
  • Phagocytes attach themselves to the surface of the pathogen
  • They engulf the pathogen by endocytosis-invagination of plasma cell membrane to form a phagosome.
  • Lysosomes (containing lytic enzymes) move towards the vesicle and fuse with it
  • The lytic enzymes are released into the phagosome.
  • The lytic enzymes within the lysosomes break down the pathogen. The process is the same as that for the digestion of food in the intestines, namely the hydrolysis of larger, insoluble molecules into smaller soluble ones.
  • The soluble products from the breakdown of the pathogen are absorbed into the cytoplasm of the phagocyte.
  • Phagocyte also displays antigenic componets on external surface of plasma cell membrane (antigen presentation) to start immune response.
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Diagram of phagocytosis


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Antigens: An antigen is any part of an orangisms or substance that is recongised as non-self (foreign) by the immune system and stimulates a immune response. They are usually proteins thar are part of the cell-surface membrane or cells walls of invading cells.

Lymphocytes: The body has specific responses that react to an indivdual forms of infection. Theses are slower but provide long-term immunity. This specific immune response depends on a type of white blood cells called a lymphocyte. The two types are:

  • B Lymphocytes (B cells)- Humoral response, matures in the bone marrow, produces antobodies, responds to foreign material outside body cell and responds to bateria and virus
  • T Lymphocyte (T cells)- Cell mediated response, matures in the thymus gland, responds to foreign material inside body cells, responds to own cells alteredby viruses or cancer and to transplant tissue

Both types are formed from stem cells found in the bone marrow:

  • B lymphocytes matures in the Bone marrow
  • T lymphocytes matures in the Thymus gand.
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Cell-mediated immunity

What is cell-mediated immunity?

T lymphocytes respond to an organism's own cells that have been invaded  by a non-self material e.g. virus or cancer cell. They also respond to transplanted material, which is generally different. They can distingish between invader and normal cells because:

  • Phagocytes that have engulfed and broken down a pathogen present some of the pathogen's antigens on their own cell-surface membrane
  • Body cells invaded by a virus present some of the viral antigens on their own cell-surface membrane
  • Cancer cells likewise present antigens on their cell-surface membranes

They are called antigen-presenting cells because they can present antigens of other cells on their own cell-surface.

As T cells will only respond to antigens that are attached to a body cells this type of response is called cell-mediated immunity.

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How cell-mediated immunity works

The stages in the response of T cells to infection by a pathogen:

  • The pathogens invade body cells or are taken in my phagocytes
  • The phagocyte places antigens from the pathogen on its cell-surface membrane
  • Receptors on certain T helper cells fits exactly onto these antigens
  • This activates other T cells to divide rapdily by mitosis and form a clone
  • The cloned T cells:

(a) Develop into memory cells which circulate the blood and tissue fluid that enable a rapid response to future infections by the same pathogen

(b) Stimulate phagocytes to engulf pathogens by phagocytosis

(c) Stimulate B cells to divide

(d) kill infected cells by producing a protein to make holes in their cell-surface membrane. These holes mean the cells becomes permable to all substances and dies.

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Diagram of cell-mediated immunity


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Humoral Immunity

What is humoral immunity?

Humoral immuity involves antibodies and antibodies are soluble in the blood and tissue fluid of the blody. There about 10 million B cells and each produce a different antibody that responds to one specific antigen.

When a antigen enters the blood there will be one type of B cell that has an antibody on its surface whose shape will fit the antigen. The antibody therefore attaches to this complementary antigen. This type of B cell divides by mitosis to form a clone of an identical B cells, all of which produce an antibody that is specific to the foreign antigen.

A pathogen has many different proteins on its surface , all of which act a antigens. Some produce toxins which act as a antigen. For each clone, the cells produced develop into one of two types of cell:

Plasma cells- secrete antibodies-2000 antibodies a second- Primary immune response

Memory cells- Circulate in the blood and tissue fluid- Secondary immune response

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How humoral immunity works

The stages of B cells in humoral immunity:

  • The surface antigens of the invading pathogen are taken up by B cells
  • The B cells process the antigens and present them on their surfaces
  • T helper cells attach to the processed antigens on the B cells thereby activating them
  • The B cells are now activated to divide by mitosis to give a clone of plasma cells
  • The cloned plasma cells produce antibodies that exactly fit the antigens on the pathogen and destroy them. This is the primary response.
  • Some B cells develop into memory cells. These can respond to future infections by the same pathogen by dividing rapidly and developing into plasma cells that produce antibodies. This is the secondary response.
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Diagram of humoral immunuty


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Antigenic variability

What is antigenic variability?

Antigenic variability= The antigens that viruses are made of and produce which are constantly changing.

  • Antigens on the surface of pathogens activate the primary response. When you're infected a second time with the same pathogen they activate the secondary response.
  • However some pathogens can change their surface antigens. 
  • This means that when you're infected for a second time, the memory cells produced from the first time will not recongise the different antigens. So the immune system has to start from scratch and carry out primary ressponse against the new antigens.
  • Antigenic variation makes it difficult to develop vaccines.

Examples of antigenic variability:

Influenza virus (flu) have over 100 different strains.This is because the proteins on the surface of the influenza acts as antigens triggering the immune system. These antigens can change regulary forming new strains. This means you will suffer from flu more than once- each time with a new strain.

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Antibodies: They react with antigens on the surface on a non-self material by binding to them precisely.

The structure of an antibody-

Antibodies are made up from four polypeptide chains. The chains of one pair are long and are called heavy chains, while the other pair are shorter and are known as light chains.

The antibody can change shape by moving as is if they had a hinge at the fork of a y-shape. Antibodies have a binding site that fits around  the antigens to form a antigen-antibody complex.

The binding site is different on different antibodies and is therefore known as the variable reigon. Each sites consists of a sequence of amino acids that form a 3D shape that binds directly to antigens. The rest of the antibody is the same and is known as the constant reigon.

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Diagram of an antibody


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Monoclonal antibodies

Monoclonal antibodies are antibodies produced from a single group of genetically identical B cells and can be isolated- they are all identical in structure

Each antigen will induce a different B cell to mulitply and clone itself. Each of these clones will produce a different antibody known as polyclonal antibodies.

Examples of how monoclonal antibodies arre used in science:

Separation of a chemical from a mixture

Immunoassay- calculating the amount of a substance in a mixture. For example pregnacy tests, testing for drugs in the urine of athletes and detecting human immunodeficiency virus (AIDS test)

Cancer treatment- Monoclonal antibodies can be made that attach themselves only to cancer cells. They can be used to actiavte cytotoxic drug. The cancer will then be destroyed causing little damage to other cells

Transplant surgery- Transplanted organs will suffer rejection because of the action of T cells. Monoclona antibodies can be used to 'knock out' these specfic T cells.

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What is vaccination?

Vaccination is the introduction of a substance into the body with the intention of stimulating active immunity against a particular disease.

Types of vaccination

  • Passive immunity- produced by the introduction of antibodies into individuals from an outside source. As the antibodies are not being produced by the individual themselves, they are not replaced when they are broken down in the body so are short-lived.
  • Active immunity- produced by stimulating the production of antibodies by the individuals own immune system. It is generally long-lasting.
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Features of a sucessful vaccination programme:

  • Economically avaliable
  • Few side effects
  • Herd immunity

Why vaccination doesn't eliminate disease:

  • Fails to induce immunity in certain individuas
  • Indiciduals may develop disease after vaccination but before their immunity levels are  high enough to prevent it
  • Pathogen may mutate
  • Varieties of a particular pathogen
  • Pathogens can 'hide' from bodys immune system
  • Individual may have regection to vaccination such as religious, ethical or medical reasons
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