Download Click here to the guide.

Composition of the Immune System
The ability of the immune system to mount a response to disease is dependent on many
complex interactions between the components of the immune system and the invading
antigens, or disease-causing agents.
Macrophages
White blood cells are the main components of the immune system. Some white blood
cells, known as macrophages, play a function in immunity by surrounding, ingesting,
and destroying invading bacteria and other foreign organisms in a process called
phagocytosis (literally, "cell eating"), which is part of the inflammatory reaction.
Macrophages also attach to invading antigens and deliver them to be destroyed by
other components of the adaptive immune system.
Lymphocytes
Lymphocytes are specialized white blood cells whose function is to identify and destroy
invading antigens. All lymphocytes begin as "stem cells" in the bone marrow, the soft
tissue that fills most bone cavities, but they mature in two different places. Some
lymphocytes mature in the bone marrow and are called B lymphocytes. B lymphocytes,
or B cells, make antibodies, which circulate through the blood and other body fluids,
binding to antigens and helping to destroy them. Other lymphocytes, called T
lymphocytes, or T cells, mature in the thymus, a small glandular organ located behind
the breastbone. Some T lymphocytes, called cytotoxic (cell-poisoning) or killer T
lymphocytes, directly destroy cells that have specific antigens on their surface that are
recognized by the killer T cells. Helper T lymphocytes, a second kind of T lymphocyte,
regulate the immune system by controlling the strength and quality of all immune
responses.
Antibodies
Antibodies are Y-shaped proteins called immunoglobulins (Ig) and are made only by B
Lymphocytes. The antibody binds to the antigen at the ends of the arms of the Y. The
area at the base of the Y determines how the antibody will destroy the antigen. This
area is used to categorize antibodies into five main classes: IgM, IgG, IgA, IgD, and IgE.
Antibodies can sometimes stop an antigen's disease-causing activities simply by
neutralization-that is, by binding the antigen and preventing it from interfering with the
cell's normal activities.
Antibodies also help destroy antigens by coating the surface of the antigens, preparing
them for ingestion by macrophages. Since macrophages have receptors that stick to the
base of the antibody's Y structure, antigens coated with antibodies are more likely to
stick to the macrophages and be ingested.
Adapted from “Blood – The River of Life,” at http://library.thinkquest.org/C0115080/?c=blood
Immune Deficiency
Deficiencies in immune function may be either inherited or acquired. Inherited immune
deficiencies usually reflect the failure of a gene important to the generation or function
of immune system components.
Some inherited diseases damage a person's innate immunity by making macrophages
incapable of ingesting or breaking down invading organisms. Individuals affected by
these diseases are especially susceptible to opportunistic infections-that is, infections
by normally harmless organisms that can flourish in a person whose immune system
has been weakened.
Adapted from “Blood – The River of Life,” at http://library.thinkquest.org/C0115080/?c=blood