Topic 5.4:Defence against Infectious diseases

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5.4.1 Skin and Mucous, barriers to infection

As a first line of defence the body has many mechanism to try to stop microbes entering the body, particularly the blood-stream. These are :

  • The skin is a tough, impenetrable barrier (which is why we use it to make leather shoes). The outer layer, the epidermis, is 20-30 cells thick (about as thick as a sheet of paper) and its cells are toughened by the protein keratin. The next layer, the dermis, is 20-40 times thicker and provides the main structure for the skin as well as all the receptor cells, blood vessels and hairs. Cells are constantly being lost from the surface of the skin (to form dust) and are replaced by new cells from further down.
  • The respiratory tract is another potential entry route, but it is protected by sticky mucus secreted by glands in the bronchi and bronchioles, which traps microbes and other particles in inhaled air before they can reach the delicate alveoli. Mucus contains lysozymes, and cilia constantly sweep the mucus upwards to the throat, where it is swallowed so that the microbes are killed by the stomach acid.
 
5.4.2 Phagocytic leucocytes

The second line of defence is the non-specific immune system, a host of quick, non-specific methods of killing microbes that have passed the first line of defence and entered the body. Phagocytic leucocytes are an example of this type of defence from infection.

phagocyte  
  • Phagocytes are large, irregularly-shaped leukocyte cells that remove bacteria, vi-ruses, cellular debris and dust particles.
  • The phagocytes are constantly changing shape, and they flow over microbes, surrounding and ingesting them through the process of phagocytosis to form a phagosome.
  • The phagosome then fuses with lysosomes - small vesicle containing lysozymes, which are released into the phagosome, killing and digesting the microbe.
  • Different phagocyte cells work in different locations: neutrophils circulate in the blood, while macrophages are found in lymph, tissue fluid, lungs and other spaces, where they kill microbes before they enter the blood.
 
5.4.3 Antigen and Antibody

 

Antigen is often used to describe something that has infected the body. However it is more accurate to describe them as follows:

  • An antigen is a large molecule (protein, glycoprotein, lipoprotein or polysaccharide) on the outer surface of a cell.
  • All living cells have these antigens as part of their cell membrane or cell wall.
  • The capsid proteins of viruses and even individual protein molecules can also be classed as antigens.
  • Their purpose is for cell communication, and cells from different individuals have different antigens, while all the cells of the same individual have the same antigens.
  • Antigens are genetically controlled, so close relative have more similar antigens than unrelated individuals.
  • Blood groups are an example of antigens on red blood cells, but all cells have them.
 

Antibodies are proteins secreted from lymphocytes that destroy pathogen and antigen infections

  • B-cells make antibodies.
  • An antibody (also called an immunoglobulin) is a protein molecule that can bind specifically to an antigen.
  • Antibodies all have a similar structure composed of 4 polypeptide chains (2 heavy chains and 2 light chains) joined together by strong disulphide bonds to form a Y-shaped structure.
  • The stem of the Y is called the constant region because in all immunoglobulin's it has the same amino acid sequence, and therefore same structure.
  • The ends of the arms of the Y are called the variable regions of the molecule because different immunoglobulin molecules have different amino acid structure and therefore different structures.
  • These variable regions are where the antigens bind to form a highly specific antigen-antibody complex, much like an enzyme-substrate complex.
  immunoglobulin    

  • Each B-cell has around 10 5 membrane-bound antibody molecules on its surface and can also secrete soluble antibodies into its surroundings.

  • Every human has around 108 different types of B cell, each making antibodies with slightly different variable regions.

  • Between them, these antibodies can therefore bind specifically to 108 different antigens, so there will be an antibody to match almost every conceivable antigen that might enter the body.

 
 
5.4.4 Antibody Production
clonal selection

(a) There are many different lymphocytes.

(b) The antigen infects and is presented to the lymphocytes

(c) The lymphocyte with a surface epitope complementary to the antigen is selected.

(d) The Lymphocyte clones to produce many plasma cells. This occurs in the lymph nodes.

(e) The clone of plasma cells

(f) The gene for the antibody is expressed and secreted into the plasma and tissue fluid.

(g) The antibody circulated in body fluids destroying the infectious antigen

 
5.4.5 The effects of HIV on the immune system
hiv T Helper

HIV is a virus that selectively infects Lymphocytes

(a) Different lymphocytes

(b) HIV virus

(c) Infection as the virus attaches then enters the host lymphocyte.

(d) The infected lymphocyte is 'disabled' by the virus

(e) When an antigen infection is presented the lymphocyte cannot produce antibodies.

(f) The antigen is not challenged by the immune system and is able to freely proliferate

The consequence is that the infected individual will develop that disease and have no immune response .

 

 

  • Therefore an individual who is HIV +ve (infected ) will eventually develop a disease which will go unchecked. The consequence is that that disease will severely damage the infected person and will eventually bring about their death.

BBC 4 ' Search for Immunisation'

pdf article (excellent)