Immunity: active, passive, and delayed WORLD OF MICROBIOLOGY AND IMMUNOLOGY288•
ent antigens and antibodies in serum. Immunobiology also
advanced. Frank Macfarlane Burnetsuggested that animals did
not produce antibodies to substances they had encountered
very early in life; Peter Medawar proved this idea in 1953
through experiments on mouse embryos.
In 1957, Burnet put forth his clonal selectiontheory to
explain the biology of immune responses. On meeting an anti-
gen, an immunologically responsive cell (shown by C. S.
Gowans (1923– ) in the 1960s to be a lymphocyte) responds
by multiplying and producing an identical set of plasma cells,
which in turn manufacture the specific antibody for that anti-
gen. Further cellular research has shown that there are two
types of lymphocytes (nondescript lymph cells): B-lympho-
cytes, which secrete antibody, and T-lymphocytes, which reg-
ulate the B-lymphocytes and also either kill foreign substances
directly (killer T cells) or stimulate macrophages to do so
(helper T cells). Lymphocytes recognize antigens by charac-
teristics on the surface of the antigen-carrying molecules.
Researchers in the 1980s uncovered many more intricate bio-
logical and chemical details of the immune system compo-
nents and the ways in which they interact.
Knowledge about the immune system’s role in rejection
of transplanted tissue became extremely important as organ
transplantation became surgically feasible. Peter Medawar’s
work in the 1940s showed that such rejection was an immune
reaction to antigens on the foreign tissue. Donald Calne
(1936– ) showed in 1960 that immunosuppressive drugs,
drugs that suppress immune responses, reduced transplant
rejection, and these drugs were first used on human patients in- In the 1940s, George Snell (1903–1996) discovered in
mice a group of tissue-compatibility genes, the MHC, that
played an important role in controlling acceptance or resist-
ance to tissue grafts. Jean Dausset found human MHC, a set of
antigens to human leucocytes (white blood cells), called HLA.
Matching of HLA in donor and recipient tissue is an important
technique to predict compatibility in transplants. Baruj
Benacerraf in 1969 showed that an animal’s ability to respond
to an antigen was controlled by genes in the MHC complex.
Exciting new discoveries in the study of the immune
system are on the horizon. Researchers are investigating the
relation of HLA to disease; certain types of HLA molecules
may predispose people to particular diseases. This promises to
lead to more effective treatments and, in the long run, possible
prevention. Autoimmune reaction, in which the body has an
immune response to its own substances, may also be a cause
of a number of diseases, like multiple sclerosis, and research
proceeds on that front. Approaches to cancer treatment also
involve the immune system. Some researchers, including
Burnet, speculate that a failure of the immune system may be
implicated in cancer. In the late 1960s, Ion Gresser (1928– )
discovered that the protein interferon acts against cancerous
tumors. After the development of genetically engineered inter-
feron in the mid-1980s finally made the substance available in
practical amounts, research into its use against cancer acceler-
ated. The invention of monoclonal antibodies in the mid-
1970s was a major breakthrough. Increasingly sophisticated
knowledge about the workings of the immune system holds
out the hope of finding an effective method to combat one of
the most serious immune system disorders, AIDS.
Avenues of research to treat AIDS includes a focus on
supporting and strengthening the immune system. (However,
much research has to be done in this area to determine whether
strengthening the immune system is beneficial or whether it
may cause an increase in the number of infected cells.) One
area of interest is cytokines, proteins produced by the body
that help the immune system cells communicate with each
other and activate them to fight infection. Some individuals
infected with the AIDS virus HIV(human immunodeficiency
virus) have higher levels of certain cytokines and lower levels
of others. A possible approach to controlling infection would
be to boost deficient levels of cytokines while depressing lev-
els of cytokines that may be too abundant. Other research has
found that HIV may also turn the immune system against itself
by producing antibodies against its own cells.
Advances in immunological research indicate that the
immune system may be made of more than 100 million highly
specialized cells designed to combat specific antigens. While
the task of identifying these cells and their functions may be
daunting, headway is being made. By identifying these spe-
cific cells, researchers may be able to further advance another
promising area of immunologic research, the use of recombi-
nant DNAtechnology, in which specific proteins can be mass-
produced. This approach has led to new cancer treatments that
can stimulate the immune system by using synthetic versions
of proteins released by interferons.See alsoAntibody and antigen; Antibody formation and kinet-
ics; Antibody, monoclonal; Antibody-antigen, biochemical
and molecular reactions; B cells or B lymphocytes; Bacteria
and bacterial infection; Germ theory of disease; Immunity,
active, passive and delayed; Immunity, cell mediated;
Immunity, humoral regulation; Immunochemistry;
Immunodeficiency; Immunogenetics; Immunologic therapies;
Immunological analysis techniques; Immunology, nutritional
aspects; Immunology; Immunosuppressant drugs; Infection
and resistance; Invasiveness and intracellular infection; Major
histocompatibility complex (MHC); T cells or T-lymphocytes;
Transmission of pathogens; Transplantation genetics and
immunology; Viruses and responses to viral infectionIMMUNITY: ACTIVE, PASSIVE, AND
DELAYEDImmunity: active, passive, and delayedActive, passive, and delayed immunity are all variations on
the operation of the immune system, whereby antibodies are
produced in response to the presence of an antigenconsidered
to be foreign.
Active immunity occurs due to the production of an
antibodyas a result of the presence of the target antigen either
as part of an intact infecting organism, or because of the intro-
duction of the specific antigen in the form of a vaccine. The
immunity is provided by an individual’s own immune system.womi_I 5/6/03 3:23 PM Page 288