3.5 - The Immune System

Physical and chemical barriers

• The body's first line of defence is a set of barriers that stop pathogens getting inside.
• These barriers work against many different pathogens, not just one, so they are described as non-specific.

Physical barriers

• Skin - Covers most of the body and acts as a physical barrier that pathogens cannot pass through. It also secretes oils and antimicrobial substances that kill pathogens on its surface.
• Nose - Contains hairs and a layer of mucus that trap particles in the air before they can travel further.
• Trachea and bronchi - The airways down to the lungs are lined with mucus that traps pathogens, and with cilia (tiny hair-like structures on the cells).
  • The cilia beat in a rhythm that wafts the trapped mucus up to the back of the throat, where it is swallowed and carried down to the stomach.

Chemical barrier

• The stomach produces hydrochloric acid, making the stomach contents strongly acidic.
• This kills most of the swallowed pathogens, whether they came in with food or in mucus from the airways.

Phagocytosis

• When pathogens get past the body's barriers and into the body, the white blood cells of the immune system destroy them.
• White blood cells travel in the blood and move into the tissues, constantly patrolling for pathogens.
• One of their main defences is phagocytosis - carried out by white blood cells called phagocytes.

How a phagocyte destroys a pathogen

1. The phagocyte tracks down a pathogen and binds to it.
2. Its flexible membrane changes shape and wraps right around the pathogen, engulfing (surrounding) it and taking it inside the cell.
3. Enzymes inside the phagocyte break the pathogen down and destroy it.

Key features of phagocytosis

• Non-specific - The same phagocyte can engulf many different kinds of pathogen, rather than acting against one in particular.
• Works alongside antibodies - Phagocytes more easily find and engulf pathogens that have been clumped together or marked by antibodies.

Antibodies, antitoxins and specificity

• As well as phagocytosis, white blood cells defend the body by producing antibodies and antitoxins.
• Every pathogen has unique molecules on its surface called antigens, and each antigen has a particular shape.
• White blood cells can tell the body's own cells from invaders:
  • Antigens on the body's own cells are recognised as "self".
  • Antigens on a pathogen are recognised as foreign, or "non-self".

How antibodies work

• When white blood cells called lymphocytes meet a foreign antigen, they start to produce proteins called antibodies.
• An antibody has a shape that is complementary to a particular antigen, fitting it like a key in a lock - so each antibody binds to only one type of antigen and won't lock onto any others.
• The lymphocyte that recognises the pathogen then divides many times to make clones of itself, all producing the same antibody - so antibodies are made rapidly, in large numbers, and are carried all round the body.
• The antibodies bind to the antigens on the pathogens. This can:
  • Destroy a pathogen directly, or
  • Clump pathogens together and mark them out so phagocytes find them more easily and engulf them.

How antitoxins work

• Some bacteria cause harm by releasing toxins - poisons that damage the body's cells.
• Antitoxins are molecules made by white blood cells that bind to and counteract these toxins, stopping them doing damage.