The immune system is a very complex interactive network of organs, tissues,
cells, and biochemical mediators designed to counteract invasion of the body by
nonself entities, such as bacteria, viruses, parasites, foreign tissue, and
other endogenous threats including malignancies. The cellular and molecular
components of the immune system recognize invading entities as nonself and mount
an effector immune response, ultimately leading to the elimination of the
foreign material. The ability of micro-organisms to cause disease depends upon
their degree of pathogenicity and the integrity of the immune response.|
An immune response is how your body recognizes and defends itself against
bacteria, viruses, and substances that look to be foreign and harmful to the
body. The immune system protects the body from potentially harmful substances by
recognizing and responding to antigens. Antigens are molecules (usually
proteins) on the surface of cells, viruses, fungi, or bacteria. Nonliving
substances such as toxins, chemicals, drugs, and foreign particles (such as a
splinter) can be antigens. The immune system recognizes and destroys substances
that contain these antigens.
Now even your own body cells have proteins that are antigens. These include a
group of antigens called human leukocyte antigens (HLAs). This is where the
immune system learns to see these antigens as normal and doesn't usually react
Innate, or nonspecific, immunity is a defense system that you are born with and
should protect you against all antigens. The innate immune response provides a
first-line, non-antigen-specific response to nonself material. Innate immunity
involves barriers that keep harmful materials from entering your body including
physical, chemical, and microbiological barriers.
Some examples of anatomical innate immunity include:
||Enzymes in tears and skin oils
||Mucus - which traps bacteria and small particles
Innate immunity also comes in a protein chemical form, called innate humoral
immunity. Examples of this would include the body's complement system and
substances called interferon and interleukin-1 (which causes fever). If an
antigen gets past these barriers, it's attacked and destroyed by other parts of
the immune system.
The white blood cells involved in innate immunity and each type having a
different function are: Monocytes (which develop into macrophages), neutrophils,
eosinophils, basophils, and natural killer cells. The complement system and
cytokines also participate in innate immunity.
Acquired immunity, also called adaptive immune response, is an immunity that
develops with exposure to various antigens over time. Your immune system builds
a defense that is specific to that antigen as well as many others. This type of
response can be characterized by antigen-specific reactions through T
lymphocytes or T cells (for cell-mediated immunity) and B lymphocytes or B cells
(for producing antibodies).
Lymphocytes are the type of white blood cell responsible for acquired immunity.
Typically, an acquired immune response begins when antibodies, produced by B
cells, encounter antigen. Dendritic cells, cytokines, and the complement system
(which enhances the effectiveness of antibodies) are also involved.
As a person's immune system encounters foreign substances (antigens), the
components of acquired immunity learn the best way to attack each antigen and
begin to develop a memory for that antigen. Acquired immunity is also called
specific immunity because it tailors its attack to a specific antigen previously
encountered. Its hallmarks are its ability to learn, adapt, and remember.
Acquired immunity takes time to develop after initial exposure to a new antigen.
However, because a memory is formed, subsequent responses to a previously
encountered antigen are more effective and more rapid than those generated by
This type of immunity is usually slow to begin a response to a new antigen, but
over several days of clonal expansion by T and B cells, the response quickens
rapidly. This type of system has a memory, so any subsequent antigen exposure is
met with a rapid response.
Passive immunity involves antibodies that are produced in a body other than your
own. Infants have passive immunity because they are born with antibodies that
are transferred through the placenta from the mother. These antibodies will
typically disappear between 6 and 12 months of age.
Passive immunization involves injection of antiserum, which contains antibodies
that are formed by another person or animal. It provides immediate protection
against an antigen, but doesn't provide long-lasting protection. Gamma globulin
given for hepatitis exposure and tetanus antitoxin are examples of passive
immunization. This is why if you have cut yourself and can't remember when your
last tetanus shot was, a doctor will give you another one while treating that
The immune system includes certain types of white blood cells. It also includes
chemicals and proteins in the blood, such as antibodies, complement proteins,
and interferon. Some of these directly attack foreign substances in the body,
and others work together to help the immune system cells.
Lymphocytes are white blood cells, which includes B and T cells.
||B cells produce antibodies that will attach to a specific
antigen and make it easier for the immune cells detect it.
||T cells attack antigens directly and help control of the
immune response. They also release chemicals, known as
interleukins, which control the entire immune response.
As lymphocytes develop, they normally learn to tell the difference between your
own body tissues and substances that aren't normally found in your body. Once B
and T cells are formed, a few of those cells will multiply and provide "memory"
for the immune system. This allows the immune system to respond faster and more
efficiently the next time you are exposed to the same antigen and in many cases
will prevent you from getting sick. So when an individual who has had chickenpox
or immunized against it, they are therefore immune from getting chickenpox
The inflammatory response or inflammation occurs when tissues are injured by
bacteria, trauma, toxins, heat, or any other cause. The damaged tissue releases
chemicals including histamine, bradykinin, and serotonin. These chemicals cause
blood vessels to leak fluid into the tissues, causing swelling. This helps
isolate the foreign substance from further contact with or spreading to other
The chemicals also attract white blood cells called phagocytes that basically
eat microorganisms and dead or damaged cells during a process called
phagocytosis. The phagocytes eventually die, and then pus is formed from a
collection of dead tissue, dead bacteria, and live and dead phagocytes.
Immune System Disorders and Allergies
Immune system or autoimmune disorders occur when the immune response is
inappropriate, excessive, or lacking. Immunodeficiency disorders would also be
an example of when the immune response is lacking. Allergies involve an immune
response to a substance that most people's bodies perceive as harmless.
Additional information can be found in the
Immune Disorders section.
Vaccination or immunization is a way to trigger an immune response. This can be
done with small doses of an antigen, such as dead or weakened live viruses, that
are given to activate immune system "memory" (activated B cells and sensitized T
cells). This memory allows your body to react quickly and efficiently to future
Immune tolerance is the tendency of T or B cells to ignore the body's own
tissues. Maintaining tolerance is important because it prevents the immune
system from attacking its fellow cells. It's still not fully understood how the
immune system knows when to respond and when to ignore an antigen.
Tolerance occurs in at least two ways—central tolerance and peripheral
tolerance. Central tolerance occurs during lymphocyte development when very
early in each immune cell's life, it's exposed to many of the self molecules in
the body. If it encounters these molecules before it has fully matured, the
encounter activates an internal self-destruct pathway, and the immune cell dies.
This process, called clonal deletion, helps ensure that "self-reactive" T and B
cells, those that could develop the ability to destroy the body's own cells,
don't mature and attack healthy tissues.
Because maturing lymphocytes don't encounter every molecule in the body, they
must also learn to ignore mature cells and tissues. In peripheral tolerance,
circulating lymphocytes might recognize a self molecule but can't respond
because some of the chemical signals required to activate the T or B cell are
absent. So-called clonal anergy, therefore, keeps potentially harmful
lymphocytes switched off. Peripheral tolerance may also be imposed by a special
class of regulatory T cells that inhibits helper or cytotoxic T cell activation
by self antigens.