When the surface of the body is punctured the blood clots to seal the wound and prevent further entry of bacteria. The mechanism for this is complex since it must not happen accidentally within the blood vessels as a floating blood clot would form and this would travel around the system and could block a small artery leading to a vital organ. On occasions such an event does occur causing an embolism which may lead to cardiac thrombosis or cerebral thrombosis. Under normal conditions the blood meets the air as it leaves a wound and this causes the platelets to release a substance called thromboplastin this activates a substance in the blood called prothrombin to convert into thrombin. For this conversion to occur Ca⁺⁺ ions, and a number of blood clotting factors must also be present. The thrombin causes a plasm protein, fibrinogen to unravel into sticky threads of fibrin. These protein chains intertwine to form a mesh which adheres to the cut tissue and traps red blood cells to form a clot. The clot then dries to form a scab. The blood contains an enzyme called antithromboplastin which breaks down thromboplastin to prevent its general dispersion into the body which would result in blood clots within distant blood vessels. The tissue below the scab then heals and repairs itself. In the condition known as Haemophilia one of the important blood clotting factors, factor VIII is absent and the blood fails to clot. As well as the clotting action the damaged tissue at the cut site releases histamine. This causes dilation of capillaries and arterioles around the cut area causing the blood to flow more slowly thereby reducing loss and increasing the chances of the clot adhering to the damaged tissue. Histamine also attracts phagocytes which leave the blood by moving out of the distended capillaries and these attack any bacteria which have entered the wound.

The immune system works by recognition of foreign substances called antigens which we must produce an antibody to. Some of these foreign materials are toxic in which case the antibody will act as an antitoxin which neutralises its poisonous effects.

When foreign organisms or material enter the body the white cells must recognise it as foreign and attack it. The lymphocytes will produce antibodies which is a form of chemical attack, while the phagocytes will attempt to engulf and digest the foreign material and digest it with their lysosomes. This system has as a basic prerequisite that the white cells can recognise our own tissue so that they do not destroy our own organs, a disease called autoimmune syndrome. Just after birth the thymus gland becomes active and the various antigen producing cells are passed through it. In the thymus there are a group of cells which display on their surface our own antigens. Any antibody producing cell which reacts with one of these display cells is immediately killed off. This removes from our system any cell capable of producing an antibody which could damage our own body cells. The remaining cells increase in number to form a basic stock of antibody producing cells which remain in small numbers within our system. If a microorganism attacks and enters our body it is broken down by a macrophage cell and its chemical components are broken down into a series of substance which are displayed by the macrophage on its surface. A group of lymphocytes called T-helper cells is constantly monitoring the macrophages and if they contact one which is displaying an antigen of a type which matches their own surface antibody they react together and a substance called interleukin I is released from the macrophage onto the T-helper cell. This causes the T-helper cell to start to divide and react with any b-lymphocyte which can produce it’s particular antibody type. The T-helper cells release interleukin II onto any b-lymphocyte they react with and this stimulates the b-lymphocytes of that type to rapidly reproduce forming a clone population. Some of them remain dormant and form what are called b-memory cells. The others begin to actively produce their antibody. The antibody is released into the bloodstream where it reacts with the antigens on the foreign organism. The antibody can have a range of effects, it may cause the cell membrane of the organism to split open, it may coat the organism and cause it to stick to any other organism which is similarly coated, or it may mark the organism for attack by phagocytes. Once the population of invading organisms begins to decline the release of interleukins declines and the T-helper cells become inactive and the b-lymphocytes decline in number. However the memory cells remain in the system for many years and should the same foreign organism invade again it will be met by an almost immediate massive antibody reaction which generally prevents a serious recurrence of the illness. The system is efficient because only those cells capable of producing the types of antibodies able to attack the antigens we actually meet in our particular environment are cloned to take up valuable space in our system, the others are present only in very tiny numbers. Unfortunately the system is not foolproof. It takes a number of days for the antibodies or antitoxins to be produced and in this time we may be dead. A further problem is that given the same antigen material different people will break it down into different individual antigens and produce different types of antibody and some of these may be very inefficient, such individuals will have very little resistance to the disease. Some parasitic bacteria, viruses and protists are able to alter the antigen material on their surface by mutation and this means that the antibody produced in response to the first attack will be useless when the mutated form appears. Some parasites can avoid detection by the macrophages which initially start the process and so avoid detection and move freely through our system causing great damage to us. When you consider the fact that a developing embryo in its mother’s womb is a genetically different parasite and it avoids detection and attack by secreting immunosuppressant substances to mask its presence to our defence system it is not surprising that other parasitic organisms have employed similar tricks.