Encyclopedia of Health and Medicine

246 The Immune System and Allergies
antibody-mediated immunity The mechanism
through which specialized immune cells, primarily
B-cell lymphocytes, carry out the IMMUNE RESPONSE
to protect the body from extracellular pathogens
(disease-causing entities, such as BACTERIA and
other microbes, present in the BLOOD or LYMPH that
have not invaded the body’s cells). Antibodymediated
immunity, also called humoral immunity,
encompasses the functions of ANTIBODY
production and immune memory. Antibody-mediated
immunity functions collaboratively with CELL-
MEDIATED IMMUNITY to help protect the body from
INFECTION.
Specialized B-cell lymphocytes called PLASMA
cells produce antibodies, protein molecules that
circulate in the blood and lymph. Exposure to
antigens for which they are sensitized activates
plasma cells to produce antibodies. The antibodies
bind with the antigens that match their ANTIGEN
receptors (molecular sites that match the configuration
of the antigen). The antibody–antigen bond
activates the COMPLEMENT CASCADE, a complex
interaction of blood proteins (the complement system)
that results in penetration and death of the
foreign cell.
Antibody binding also releases various
CYTOKINES (protein molecules that serve as chemical
messengers) that then activate other processes
in the immune response. Immune cells that may
respond to these messages include macrophages,
granulocytes (basophils, eosinophils, and neutrophils),
and T-cell lymphocytes (notably helper
T-cells [Th2 cells]). The sequence of events takes
about 36 hours to unfold after the IMMUNE SYSTEM
recognizes the antigen as foreign. The immune
response then works to neutralize the threat.
Memory B-cells circulate in the blood and
lymph for an extended period of time. They hold a
“memory” imprint of specific pathogens. When
the PATHOGEN again enters the body, the memory-B
cells remember and immediately ramp up antibody
production. This process shortcuts the usual
immune response, allowing the immune system to
mount a defense before the pathogen can initiate
an infection.
See also B-CELL LYMPHOCYTE; CELL STRUCTURE AND
FUNCTION; GRANULOCYTE; MACROPHAGE; T-CELL LYMPHO-
CYTE; VACCINE.
antigen A molecule that resides on the surface
of a cell membrane and is capable of stimulating
an IMMUNE RESPONSE. Antigen molecules are either
lipoproteins (lipid and protein) or glycolipids (lipid
and GLUCOSE). Each cell has numerous antigens
that that identify it to the IMMUNE SYSTEM. Cells
that belong to the body bear antigens that mark
them as self cells; the immune system does not
react to them. The antigens on cells that are foreign
to the body alert the immune system to the
presence of nonself cells, which activates an
immune response. Foreign or nonself antigens
cause the immune system to develop antibodies,
unique proteins (immunoglobulins) that inactivate
or destroy specific antigens.
ANTIGENS AND BLOOD TYPE
Antigens form the basis of the ABO and rhesus
(Rh factor) classification for BLOOD TYPE. Antigens
coat the cell membrane surface of erythrocytes
(red BLOOD cells) for blood types A, B, and AB.
The erythrocytes of type O blood do not have
antigens. Erythrocytes may also have Rh antigens,
designated as “positive” when used to
identify blood type. For example, A+ erythrocytes
bear type A and Rh antigens. O– erythrocytes
have neither ABO antigens nor Rh
antigens.
Antigen Processing
Macrophages, tissue-bound phagocytic white
BLOOD cells that start life as monocytes circulating
in the blood, are abundant in the LYMPH tissues.
They are the immune cells that sound the alarm
to the rest of the immune system that nonself
antigens are present. When a macrophage
encounters a foreign entity, it surrounds and
ingests it. As the MACROPHAGE consumes the
invader, it displays the invader’s antigens on the
surface of its cell membrane. This display
announces the presence of the antigens to other
immune cells, notably T-cell lymphocytes, which
then mount a full immune response. Other cells
that may serve as antigen-presenting cells include
B-cell lymphocytes and dendritic cells. Once T-cell
lymphocytes “read” the antigen message the
macrophage displays, they respond by attacking
and killing the invader. Correspondingly, B-cell