Immune system

The immune system1 of an organism is a complex biological system made up of a coordinated set of elements of recognition and defense that discriminates the self from the non-self. It is inherited at birth, but autonomous, adaptive and endowed with great plasticity, it then evolves according to the contacts it has with microbes or environmental substances foreign to the body.

What is recognized as non-self is destroyed, like pathogens: viruses, bacteria, parasites, certain “foreign” particles or molecules (including certain poisons). The immune system is responsible for the phenomenon of transplant rejection.

There are several types of immune systems among animal species, and generally several immune mechanisms work together within a single organism. Many species, including mammals, use the variant described below.

The main effectors of the immune system are immune cells called leukocytes (or white blood cells) produced by stem cells within the red bone marrow.

The immune system is made up of 3 layers:

the epithelial barrier as the protection of the skin and mucous membranes, gastric acidity; non-specific or innate or natural defense mechanisms deriving from cells of the myelocytic lineage; specific or adaptive defense mechanisms deriving from cells of the lymphocyte lineage. On the one hand, it is responsible for the secretion of proteins specifically directed towards extracellular pathogens: the antibodies produced by B lymphocytes. This immunity is called humoral immunity. On the other hand, intracellular pathogens are taken care of by T lymphocytes: this immunity is called cellular immunity. This last layer only exists in vertebrates. This separation into three layers does not prevent a very important interaction of the layers between them.

The immune response is called the activation of the mechanisms of the immune system in the face of recognition of non-self, aggressive or not, in the face of aggression or dysfunction of the organism.