Chapter 4 Homeostatic Mechanisms • MHR 127and then goes through a process of rapid cell
division (clonal expansion). This process produces
a number of different types of T cells. Helper T cells
give off chemicals that stimulate other macrophages,
B cells, and other T cells. Suppresser T cellsslow
and stop the process of cellular immunity, while
memory T cellsremain in the bloodstream to
promote a faster response if the same foreign
antigen appears again. Cytotoxic T cellsbind to
other cells that have been infected and destroy
them by puncturing a hole in their cell membrane.
These destroyed cells may be invading bacteria,
human cells that have been infected by a virus, or
cancer cells (see Figure 4.18). The cytotoxic T cells
can also be triggered directly by foreign antigens
bound to a cell that has been attacked by a virus.
Figure 4.18A scanning electron microscope shows
cytotoxic T cells attacking and destroying a cancer cell.
As shown in Figure 4.19 on the following pages,
antibody immunity begins the same way as cellular
immunity does — with a macrophage ingesting a
foreign cell and presenting antigens from that cell
on the surface of the macrophage’s cell membrane.
In this case, helper T cells trigger specialized
B cells (that recognize the antigen) to undergo
rapid cell division. The dividing B cells form
plasma cells and memory B cells. The plasma cells
produce large quantities of antibodies that are
matched to the foreign antigen. These antibodies
attach to the foreign cells, which are ingested by
macrophages. Antibodies can also attach to toxins,
trigger the release of other defensive chemicals
from surrounding cells, and prevent foreign cells
from attaching to the digestive or respiratory
systems. When the danger has passed, the plasma
cells undergo apoptosisor programmed cell death.
As in the case of memory T cells, memory
B cells stay in the body to speed up the response
if the same antigen reappears. Vaccines contain
antigens that trigger the immune system to respond,
thus protecting us from future infection of a
particular disease.Allergies — A Good
System Gone Bad
An allergy is an exaggerated response by the immune
system to a harmless material such as pollen, mould,
or cat dander. There are two major types of allergic
reactions: immediate and delayed. An immediate
or acute reaction is the most common type of
allergic reaction. It occurs within seconds of
exposure to the allergen and usually disappears
within 30 minutes. In these reactions, specialized
antibodies trigger certain cells to release histamines,
which are chemicals that increase the permeability
of blood vessels, making the area red and swollen.
The specialized antibodies can also trigger the
release of cellular fluids, which can result in
watery eyes and a runny nose.
Food allergies can trigger vomiting, cramps, and
diarrhea, although most of these symptoms can be
treated with antihistamines.
Some forms of asthma, the most common chronic
disease among North American children, is an acute
reaction to allergens that are inhaled. The inhaled
allergens trigger a massive release of histamines,
which sets off spasms of the bronchioles, the tiny
air passageways in the lungs. In people who have
asthma, these passageways are particularly
sensitive. Spasms can also be triggered by stimuli
such as cold air and fatigue. The result can be
coughing, wheezing, and sometimes fatal suffocation.
Asthma can be treated with anti-inflammatory
drugs such as various steroids, and with
bronchodilators. The latter contain medicines that
can open the airways of the bronchia and thereby
ease the symptoms of an acute attack. Current
research is exploring new drugs that may provide
long-term relief from the inflammation that1 μmT cellstarget cell