You can catch a communicable disease when you are exposed to a pathogen. There are many ways in which this can happen, but some include touching an infected person, drinking dirty water or breathing in an airborne pathogen.
If pathogens pass the non-specific first line of defence they will cause an infection. However, the body has a second line of defence to stop or minimise this infection. This is called the immune system. As a part of this there are two types of white blood cell called phagocytes and lymphocytes.
Phagocytes are white blood cells. They are attracted to pathogens. They surround them in the blood, bind to them and engulf them.
The phagocytes' membrane surrounds the pathogen and the enzymes found inside the cell, then break down the pathogen in order to destroy it. As phagocytes do this to all pathogens that they encounter, we call them 'non-specific'.
Lymphocytes are another type of white blood cell. They recognise proteins on the surface of pathogens called antigens. Lymphocytes detect that both the proteins and pathogens are foreign, not naturally occurring within your body and produce antibodies. This can take a few days, during which time you may feel ill. The antibodies created by the lymphocytes cause pathogens to stick together, and make it easier for phagocytes to engulf them.
A specific type of lymphocyte called a memory lymphocyte can 'remember' the antigens from an infection by a previous pathogen. A second exposure to it will result in a much faster immune response. Antibodies will be produced much faster, which often stops us becoming ill again. There are hundreds of common colds caused by different viruses. It is very unlikely you will become infected by the same virus because memory lymphocytes exist to fight the infection immediately. This response to a known antigen is called the secondary response and it is much quicker than the response to an antigen for the first time.
During the primary infection the antibodies slowly increase, peak at around ten days and then gradually decrease. A second exposure to the same pathogen causes the white blood cells to respond quickly in order to produce lots of the relevant antibodies, which prevents infection.
Some pathogens produce toxins which make you feel ill. Lymphocytes can also produce antitoxins to neutralise these toxins. Both the antibodies and antitoxins are highly specific to the antigen on the pathogen, thus the lymphocytes that produce them are called 'specific'.