Functions

There are two important functions of the immune system that are; recognition of an invading pathogen  disease-producing  fungi,  bacteria, viruses and protozoa as  being distinct from normal body components and thus treated as foreign and then the triggering of pathways that lead to destruction of the invader, such as activation of  complement and  phagocytic  cells  which  engulf  and  digest  the invading organism. The immune system may also be able to destroy and recognize abnormal cells that arise spontaneously in the body that would otherwise lead to cancer but the signi?cance of this phenomenon in protecting against human tumors is hence debatable. The key cells responsible for immunity in vertebrates are white blood cells known as lymphocytes that arise from precursor stem cells in the bone marrow. There are two major parts of the immune system that interact to gives whole protection for the animal:

-  The  humoral  immune  response  (humor  is  an  ancient  word  which mean  ?uid) relies on the production of soluble proteins known as antibodies or immunoglobulins through B lymphocytes  so called hence the cells mature  in the bone marrow. As a general shorthand nomenclature, B lymphocytes are frequently called simply B cells.

- The  cellular  immune  response  is  mediated  through  T  lymphocytes,   so  called hence  their  maturation  from  stem  cells  happens  in the  thymus.  In the cellular immunity it is the intact T lymphocytes themselves which are responsible for the recognition and killing of foreign invaders. These cells are the CTL (cytotoxic T lymphocytes), also called killer T cells.  Other T lymphocytes have another role; they give essential help for B lymphocytes to produce anti- bodies and so are called helper T cells.

In both humoral and cellular immunity, recognition  of the  foreign  invader depends  upon  the  recognition  of  foreign  macromolecules   (proteins,  carbohydrates, nucleic acids); these foreign elements are called antigens.