Immune system



The immune system of a multicellular organism acts as a defense against
pathogens, such as viruses, bacteria, parasites, and some poisons. There are
several variations of immune systems throughout species, and sometimes more
than one immune system within the same organism (for example, the human
brain has its own immune system that is separate from the "normal" one).

The Latin term immunis means exempt, referring to protection against foreign
agents. The recognition of what is foreign is found in all life. In
self-pollinating plants, a pollen grain landing on the stigma of a flower
will send a pollen tubule down the style to the ovary for fertilization. A
pollen grain from a genetically distinct plant will not germinate or the
pollen tubule, once formed, will disintegrate in the style. In
cross-pollinating species, self-marked pollen grains disintegrate, while
nonself grains germinate and fertilize.

We may conceive of an arrangement where the cells of self are marked, so
that they are not attacked by its own defense mechanism. But not all foreign
cells may be destroyed since some must be assimilated for nourishment.
Therefore, the immune system must have the capacity to detect self and some
nonself. But since self needs to assimilate some nonself for its survival,
it cannot mark itself. It is easier to mark potentially dangerous selves.
But if only certain nonselves are marked, how does the body prepare to
defend itself from selves not seen? The defense system must have the
capacity to transform itself to deal with future dangers. It must also have
the capacity to change, since the self itself evolves with time.

The immune system is based on immune cells called leukocytes (or white blood
cells) that are produced by stem cells in the bone marrow. The immune system
can be divided into two parts. Many species, including mammals, have the
following type:

   * The humoral immune system, which acts against bacteria and viruses in
     the body liquids (such as blood). Its primary means of action are
     immunoglobulins, also called antibodies, which are produced by B cells
     (B means they develop in the bone marrow).
   * The cellular immune system, which takes care of other cells that are
     infected by viruses. This is done by T cells, also called T lymphocytes
     (T means they develop in the thymus). There are two major types of T
     cells:
        o Cytotoxic T cells (TC cells) recognize infected cells by using
          T-cell receptors to probe the surface of other cells. If they
          recognize an infected cell, they signal the cell to "commit
          suicide", thus killing that cell and any viruses it is in the
          process of creating.
        o Helper T cells (TH cells) interact with macrophages (which ingest
          dangerous material), and also produce cytokines (interleukins)
          that induce the proliferation of B and T cells.

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