How the Body Works: The Immune Mechanism
The immune mechanism is designed to protect the body against attack
from invading microorganisms and foreign, potentially harmful
molecules. There are four types of immune mechanisms. The nature of
the invading antigen determines which type of mechanism is brought
into action. Certain antigens promote an exaggerated response,
called a hypersensitive reaction, or an allergy., which may be
harmful to the body tissues.
A type I reaction is an allergic response to foreign substances,
usually proteins, entering the body. It is an immediate reaction
which occurs within minutes or hours of the antigen entering the
body. The diagram follows the events that occur in a type I
response. The antigens enter the body and stimulate B-lymphocytes to
produce antibodies. The antibodies then adhere to mast cells in the
vessel wall. They neutralize the antigens and the mast cells release
a chemical which causes, for example, the streaming eyes and the
sneezing symptomatic of hay fever.
A type II reaction is initiated by antigens which are part of, or
closely associated with, a tissue cell. The diagram shows antigens
entering the bloodstream and invoking the production of antibodies.
The antibodies destroy the antigens, but they may also cause, for
example, a cross reaction with blood cells which can lead to cell
damage. An example of this type of reaction is a mismatched blood
transfusion, in which antibodies are formed against the donor red
cells, which leads to their destruction.
A type III reaction is an immediate reaction occurring within a few
hours of a small antigenic stimulation. The diagram shows antigens
entering blood already filled with antibodies, formed during a
previous exposure to these antigens. The antibodies form a complex
with the antigen and a blood protein called complement. The complex
so formed may damage tissue, such as the glomeruli of the kidneys,
by blocking up the capillaries.
The final type of reaction, the type IV reaction, is a delayed
immune response which occurs more than twenty-four hours after the
initial contact with the antigen. The antigens enter the
bloodstream, where they stimulate T-lymphocytes to produce
antibodies which remain attached to the cell wall. The antibodies
then destroy the antigens. Once the T-lymphocytes have been
sensitized by the antigen, they can produce antibodies and confer
immunity. This is the basis of immunization against tuberculosis.