Immune System Mechanisms: organs, cells, antibodies

The human immune system is a complex and evolving defensive apparatus, essential for protection from invading pathogenic micro-organisms (bacteria, fungi, protozoa, viruses) and from the numerous other substances regularly trying to invade our body. The immune system is able to discriminate between “own” (self) substances and “alien” (non-self) ones. Alien substances have molecules called antigens on their surface, which are recognized by the immune system and therefore attacked.

The mechanisms of the immune system are antigen-specific (able to recognize and attack particular antigens), systematic (not limited to the original place of infection, but acting in the whole body) and have "immunological memory" (they recognize an antigen that has already invaded the body and organize an aimed defence).

Immune system organs

The immune system is composed by a complex range of highly specialized organs and cells, distributed throughout the body, cooperating, each in its own role, for the body’s defence.

Lymph nodes and lymphatic vessels belong to a particular circulatory system transporting lymph, a transparent fluid containing mainly white blood cells called lymphocytes. They are like "stations" where the immune system cells can reproduce in order to fight a specific alien agent.

The spleen, in the left upper part of the abdomen, is another collecting place where the lymphatic cells transport alien organisms carried by the lymphatic system. Lymphatic tissues can also be in other parts of the body and of organs, like bone marrow, thymus, tonsils, adenoids and the appendix.

Immune Cells

The immune system acts through particular cells, lymphocytes and phagocytes, which are able to recognize harmful agents and attack them. There are two particular classes of lymphocytes: B lymphocytes, developing in the bone marrow, and T lymphocytes maturing in the thymus.

B-cells are cells matured by bone marrow creation, and work through the secretion of chemical molecules, called antibodies, into surrounding tissue. These antibodies link themselves to the antigen - which the B-cells try to seek out and destroy - and mark it for destruction through chemical processes.
T-cells differ from B-cells. The difference is that they are created as immature cells, and sent to the thymus to be programmed for the recognition of a specific antigen. Another difference is that a T-cell kills cells that it has been programmed to attack through direct contact, linking itself to the cell and then injecting strong chemicals into the target cell.
There are two main types of T-lymphocytes: regulators, necessary to orchestrate the immune system by coordinating various kinds of cells, each one with very precise tasks, and cytotoxic, T-lymphocytes able to become "killer cells”, which attack and destroy the infected cells.

B-cells are free to circulate in blood, but they cannot enter living tissue. On the contrary, a T-cell can move anywhere in the body. T-cells can also assist B-cells by triggering a chain reaction making B-cells to form plasma cells.
These plasma cells then begin to produce more B-cells thus allowing the number of antibodies to increase exponentially. Other T-cells then have the job of monitoring the level of antigen and stopping the production of B-cells when the infection has been destroyed.

B lymphocytes and antibodies are a part of the so-called humoral immunity. T-lymphocytes instead, are a part of the so-called cell-mediated immunity, and are in the position to regulate and coordinate the entire immune system.

The cell-mediated immunity is mediated by T-cells and its direct lyses of target (infected) cells: production of cytokines activating infected cells to kill pathogens.
The humoral immunity is mediated by antibodies produced by B-cells: antibodies bind to the whole or fractions of antigens outside cells.