All cells have proteins and other substances projecting from their cell membrane

These are known as antigens and are specific to that type of cell

Lymphocytes have the ability to ‘read’ the antigens on the surfaces of cells and recognise any that are foreign

They then make antibodies which are a complementary shape to the antigens on the surface of the pathogenic cell

The antibodies attach to the antigens and cause agglutination (clumping together)

This means the pathogenic cells cannot move very easily

At the same time, chemicals are released that signal to phagocytes that there are cells present that need to be destroyed

The initial response of a lymphocyte encountering a pathogen for the first time and making specific antibodies for its antigens can take a few days, during which time an individual may get sick

Lymphocytes that have made antibodies for a specific pathogen for the first time will then make ‘memory cells’ that retain the instructions for making those specific antibodies for that type of pathogen

This means that, in the case of reinfection by the same type of pathogen, antibodies can very quickly be made in greater quantities and the pathogens destroyed before they are able to multiply and cause illness

This is how people can become immune to certain diseases after only having them once

It does not work with all disease-causing microorganisms as some of them mutate fairly quickly and change the antigens on their cell surfaces

Therefore, if they invade the body for a second time, the memory cells made in the first infection will not recall them as they now have slightly different antigens on their surfaces (e.g. the cold virus)