Immunology is a complicated subject, and a detailed
discussion of it is beyond the scope of this text. However, an
understanding of the basic function of the immune system
is useful in order to understand both how vaccines work and
the basis of recommendations for their use. The description
that follows is simplified. Many excellent immunology
textbooks are available to provide additional detail.
Immunity is the ability of the human body to tolerate the
presence of material indigenous to the body (“self”), and to
eliminate foreign (“nonself”) material. This discriminatory
ability provides protection from infectious disease, since
most microbes are identified as foreign by the immune
system. Immunity to a microbe is usually indicated by the
presence of antibody to that organism. Immunity is generally
specific to a single organism or group of closely related
organisms. There are two basic mechanisms for acquiring
immunity, active and passive.
Active immunity is protection that is produced by the
person’s own immune system. This type of immunity is
usually permanent.
Passive immunity is protection by products produced by
an animal or human and transferred to another human,
usually by injection. Passive immunity often provides effective
protection, but this protection wanes (disappears) with
time, usually within a few weeks or months.
The immune system is a complex system of interacting cells
whose primary purpose is to identify foreign (“nonself”)
substances referred to as antigens. Antigens can be either
live (such as viruses and bacteria) or inactivated. The
immune system develops a defense against the antigen.
This defense is known as the immune response and
usually involves the production of protein molecules by B
lymphocytes, called antibodies (or immunoglobulins), and of
specific cells (also known as cell-mediated immunity) whose
purpose is to facilitate the elimination of foreign substances.
The most effective immune responses are generally
produced in response to a live antigen. However, an antigen
does not necessarily have to be alive, as occurs with infection
with a virus or bacterium, to produce an immune
response. Some proteins, such as hepatitis B surface antigen,
are easily recognized by the immune system. Other material,
such as polysaccharide (long chains of sugar molecules that
make up the cell wall of certain bacteria) are less effective
antigens, and the immune response may not provide as
good protection.