Many animals have a special group of cells that respond to infections by
a variety of defenses. Antibody-mediated immunity involves a group of
proteins called immunoglobins, which combine with foreign substances
(antigens) to inactivate the invaders. In cell-mediated immunity,
phagocytic cells are activated, which then seek out, engulf, and destroy
the infected host cells.
- Immune response: a defense system based on special cells that
respond to infection by multiplying and fighting the infection. All
vertebrates (and a few invertebrates) show immune responses; those of
mammals are the most elaborate.
- Antigen: any substance capable of evolking an immune response.
Immune responses are very specific to particular antigens.
- Primary response: The response to an initial encounter is slow,
and only moderate numbers of defensive cells are recruited.
- Secondary response: Subsequent encounters with the same antigen
produce a much more rapid response and a much stronger reaction using
many more defensive cells.
- Memory: The immune system recognizes specific antigens that
it has encountered before, so primary and secondary responses differ.
- Antibody-mediated immunity (AMI): Antibody-mediated immunity
is most effective against bacterial infections. Certain white blood cells
(B-lymphocytes or B-cells) produce special infection-fighting
proteins called antibodies or immunoglobins(Ig). There are
several classes of antibodies:
- Ig-G (about 85% of total) is Y-shaped, with 2 light and
2 heavy chains; inactivate bacteria or toxins by clumping them.
- Ig-M (13% of total) clumps bacteria and also activates
complement (see below).
- Ig-A is present mostly in saliva, tears, and mucous; it
protects body surfaces.
- Ig-D is most abundant in newborn individuals.
- Ig-E induces mast cells to release histamine,
producing inflammation.
- Antigen-antibody complexes: All antibodies disable antigens
(or cells bearing antigens) by combining with them to form insoluble
cross-linked complexes or clumps. The antigen-antibody complex also
activates complement (a group of proteins that activate phagocytosis),
causes inflammation (histamine reaction), and causes rupture
(lysis) of cells.
- Cell-Mediated Immunity (CMI): Certain antigens activate a response
that kills diseased body cells, not just the disease agents. Cell-mediated
immunity is most effective against viral infections, certain eucaryotic infections,
and some cancers.
- Major Histocompatibility Complex (MHC): cell surface proteins
that allow the body to recognize its own cells. Cells of the CMI system
attack only cells bearing both the MHC and certain specific
antigens indicating that those cells have become infected.
- Macrophage: A type of white blood cell that engulfs infected cells
and kills them by phagocytosis. It also acts as an antigen-presenting cell
(APC) that displays antigens on its surface and thus attracts
and stimulates lymphocytes.
- T-lymphocytes (T-cells) and their roles:
- Cytotoxic ("killer") T-cells attack infected cells
by making holes in their cell membranes.
- Supressor T-cells turn other lymphocytes off when they are
no longer needed.
- T-helper cells activate macrophages by releasing certain
chemicals (lymphokines) such as interleukin.
- Disorders of the immune system:
- Autoimmune disorders: the immune system attacks its own
body, as in rheumatoid arthritis, rheumatic fever, systemic lupus
erythematosus, or juvenile diabetes.
- Allergy: sensitivity to common environmental antigens
(allergens) such as ragweed pollen.
Reactions usually involve Ig-E.
- Immunodeficiency: Inability to make enough lymphocytes.
Example: Acquired Immune Deficiency Syndrome (AIDS)
results from a viral attack on the T-helper cells.
- Monoclonal antibody technique: Medical laboratory technique
that makes large quantities of antibodies against a particular antigen.
- Antigen is injected into a mouse, which develops antibodies against it.
- Spleen is removed from mouse and B-cells isolated from it.
- The B-cells are fused with fast-growing myeloma cells
from a tumor.
- Successfully fused hybridoma cells are grown on a medium
that kills other cell types.
- Hybridoma cells grow rapidly, producing lots of one specific antibody.
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