Pathogens and the immune system

Every day you meet millions of pathogens in your daily life – why aren’t you constantly ill? The human body has many different defence mechanisms against pathogens. They all depend on recognising foreign material and destroying or, at the very least, inactivating the pathogen.

• The non-specific responses of the body react to any invading pathogen.
• The specific immune responsesspecific immune responses
  The immune response of the body that reacts to specific foreign cells identified by the antigens on their surface.  each respond to specific pathogens.

The human body has a number of adaptations that either prevent the easy entry of pathogens into the body or destroy pathogens as soon as they get inside, before they can cause an infection. For example:

1. The skin forms a relatively impenetrable barrier and is covered in a healthy flora of microorganisms that out-compete most pathogens
2. Tears contain the enzyme lysozyme which causes bacteriabacteria
   Single-celled organism. Has a cell wall, cell membrane, cytoplasm. Its DNA is loosely-coiled in the cytoplasm and there is no distinct nucleus. that land on the eyes to burst (lyse).
3. The stomach contains acid that destroys most pathogens that enter through the mouth
4. The gas exchange system produces mucus that traps pathogens, and is then moved out into the throat or nose by cilia beating on the lining of the tubes.
5. If the skin is broken, the blood clotblood clot
   A mass of fibrin, platelets and red blood cells which form a solid mass and stop blood loss at a wound. s and then a scab forms. This prevents you bleeding to death and prevents the entry of pathogens directly into the blood, as well as protecting the new skin as it grows.

The cell surface membranemembrane
A thin, flexible sheet-like structure that acts as a lining or a boundary in an organism. s are the site of cell identification systems. There are glycoproteinsglycoproteins
Proteins that have a carbohydrate chain attached to them. The carbohydrate chain sticks out of the outside of the cell and is part of the cell recognition system.  and glycolipidsglycolipids
Lipids that have a carbohydrate chain attached to them. The carbohydrate chain is attached to the outside of the cell and is part of the cell recognition system.  (proteins and lipids with short carbohydratecarbohydrate
Energy producing organic compounds which are made of carbon, hydrogen and oxygen. Examples of food containing carbohydrate are rice, pasta, bread and potatoes  sections attached to the molecules) and these, along with some membrane proteins, act as antigenantigen
A protein, nucleic acid, or other molecule, which can stimulate a response from the immune system. s identifying one cell to other cells. For example this system enables the cells of the immune systemimmune system
The body's natural defence mechanism against infectious diseases. to identify pathogens, cells from other organisms of the same species (eg after an organ transplanttransplant
The process of replacing a damaged or diseased organ with a healthy organ from a dead or living donor. ), abnormal body cells (eg cancer cells) and toxins produced by pathogens. It is also key in the non-specific responses of the body to invading pathogens.

Non-specific responses to infection are usually triggered either by body cells breaking down and releasing chemicals, or by pathogens that have been labelled by the specific immune systemimmune system
The body's natural defence mechanism against infectious diseases. . Many of the non-specific responses depend on the many different types of leucocytesleucocytes
White blood cells.  (white blood cells). The different cells can be recognised both by their appearance and their functions.

Lymphocytes are one of the most important types of white blood cellswhite blood cells
Cells specialised in defending the body against disease. and make up 18-42% of circulating blood cells. They play a crucial role in the specific immune response, as will be discussed in the following pages.

• Phagocytosisphagocytosis
  Energy-requiring process by which a cell takes large particles into membrane-bound vesicles. : this involves white blood cellswhite blood cells
  Cells specialised in defending the body against disease. that engulf and digest pathogens and any other foreign material in the blood and tissues. Phagocytes engulf the pathogen into a vesicle called a phagosomephagosome
  The vesicle in a phagocyte that surrounds ingested material such as a pathogen and fuses with a lysosome to provide the enzymes needed to digest it. . This fuses with a lysosome and the enzymes break down the pathogen. Phagocytes are very important in both the non specific responses to infection and the specific immune response.

  Neutrophils make up 70% of the leucocytesleucocytes
  White blood cells. in the blood, but each neutrophil can only ingest a few pathogens before it dies.

  Macrophages make up about only about 4% of the leucocytes in the blood. They can digest enormous numbers of pathogens because they can renew the digestive enzymes in their lysosomes, which neutrophils cannot do. Cells called monocytes move to infected tissues and become macrophages, so there are lots of macrophages in the tissues where they are needed.

  The pus that sometimes forms in spots or infected cuts is a build-up of dead cells, mainly neutrophils. The dead neutrophils give the pus its yellowish-white colour.