The body's first line of defense against germs is made up of skin and mucous membranes. The skin is the body's largest organ and offers incredible protection against certain infections. Despite its efficiency, certain external elements are still able to penetrate the body. These elements are recognised by the immune system as foreign (not belonging to it) and potentially dangerous (responsible for diseases or infections). Therefore, they must be eliminated. The immune system is our best natural defense system against diseases: it fights against viruses, bacteria, fungal infections and parasites.
Upon entering the body, elements that are considered to be foreign invaders (known as antigens) are attacked by the immune system's cells and organs or, in other words, our natural defenses. This immune response involves:
Diagram: Location of lymphoid organs
Most often, when a foreign element is detected by the immune system an immediate elimination response is initiated. This response involves phagocytes and leukocytes that constantly circulate in the body. This response is fast and so-called non-specific, which means that the immune system attacks the antigen without knowing its nature. This type of response results in the phenomenon of inflammation whose clinical signs include the appearance of redness, pain and heat.
Depending on the seriousness of the infection, this quick and non-specific immune response is not always sufficient to eliminate the invader. Therefore, a second slower and specific response is implemented whereby the foreign element is recognised by immune cells, particularly in the lymph nodes. Following recognition, these immune cells, which are specifically adapted to the destruction of the foreign agent (lymphocytes) will quickly multiply in the lymphoid organs. Thereafter, the body remembers this encounter with the foreign element (thanks to B lymphocytes). This is called immune system memory. This makes it possible to optimise the specific immune response, which will be quicker at the time of the next encounter with the same foreign element. This is also the principle of vaccination whereby non-pathogenic (safe) foreign agents are inoculated. The immune system learns to fight them and, by remembering them, can quickly eliminate them during subsequent encounters.