The immune system has developed in all vertebrates as a protection against infectious diseases. Invertebrates possess only a primitive defense system and must rely on phagocytic cells (macrophages and neutrophilic cells). Although such cells also have an important function in protecting vertebrate organisms against infections, they are only one component of a much more complex and refined defense strategy, the immune system. Responsible for putting it together are the lymph-associated tissues and the T- and B-lymphocyte populations that mature there.
One distinguishes between two kinds of immune responses:
- a cell-mediated immune response, for which the T- lymphocytes are responsible
- a humoral immune response of the B-lymphocytes and plasma cells, which acts by producing antibodies.
Like the nerve system, the immune system has the ability "to remember". For this reason we develop, for example, after our first contact with the appropriate virus a lifelong immunity against the diseases caused by these viruses.
Two types of differentiated cells are responsible for the defense and memory:
- Memory cells
- Effector cells
Despite many similarities, the two lymphocyte populations, the B- and the T cells, differ in several fundamental aspects:
- Distance of the effect
- Type of antigen recognition