Defensive strategies used by the immune system fighting pathogen agents are various and are mainly divided into innate immunity and acquired immunity.

Innate immunity consists of a series of non-specialised defence systems and represents the first defence barrier, which is always active (it does not require a previous exposure to the pathogenic agent) and has no immunologic memory.

Acquired immunity is the system able to effectively challenge micro-organisms with which it has already had a contact. This system has a long-term response and a continuing evolution because it is modelled on received stimuli. Moreover, it has a high efficiency because it is activated only when it is necessary and – due to immunologic memory – ensures a long-term protection.


After B and T lymphocytes activation against an alien agent, some of them transform themselves in memory lymphocytes, remaining for very long periods in the body, sometimes a long-life period, after infection. When the same antigen appears again (that is to say in case one is affected by the same infection for a second time), the immune system, due to memory cells, immediately recognizes the alien agent and so is able to eliminate it rapidly, before the spreading of the disease.

During pregnancy, some kinds of immune cells can pass through the placenta from mother to child. In this way, maternal immune memory will be useful for the newborn during the first months. This protection is called passive natural immunity and has a limited duration, because maternal cells are gradually eliminated from the newborn’s body.