THE ADAPTIVE IMMUNE SYSTEM
This system is based on white blood cells called lymphocytes. It has two parts.
Humoral immunity relies on the action of B-lymphocytes; these cells produce
antibodies, which circulate and attack specific microbes. In cellular immunity, cells
called T-lymphocytes are activated and attack specific microbes or abnormal cells
(such as virally infected cells or tumour cells).
A humoral response is started when an
antigen (foreign protein), here on the
surface of a bacterium, is recognized by one
type of B-lymphocyte and activates it.
An antigen, here on the surface of an
abnormal cell (such as a virus-infected cell
or a tumour cell), is recognized by specific killer
(cytotoxic) T-lymphocytes and activates them.
This particular type of B-lymphocyte
multiplies, forming cells called plasma
cells, which make antibodies designed
specifically to attack the bacterium.
With the assistance of helper T cells (another
type ofT-lymphocyte), the appropriate killer
T-lymphocyte begins to multiply.
After a few days, the antibodies are
released. They attach themselves to the
antigen. This triggers more reactions, which
ultimately destroy the bacterium.
Some B-lymphocytes remain in the body
as memory cells; if the bacterium enters
the body again, they rapidly produce large
amounts of antibodies to haltthe infection.
The killer T-lymphocytes attach themselves
to the abnormal cells (a), leading to the
cells’ destruction (b). The T-lymphocytes survive
and may go on to kill more targets.
Some ofthe killer T-lymphocytes remain
in the body as memory cells, and quickly
attack abnormal cells should they reappear (for
example, after reinfection with a virus).
EXAMPLES OF INFECTIOUS ORGANISMS COMBATED
Humoral immunity particularly
• Some viruses (e.g. hepatitis)
• Many bacteria (e.g. cholera)
• Some parasites (e.g. malaria)
Cellular immunity particularly
• Manyviruses(e.g. herpessimplex)
• Some bacteria (e.g. tuberculosis)
• Some fungi (e.g. candidiasis)
a group of blood proteins called the
destroy the invading organisms.
The second part of the immune sys-
tem, w hich is known as “acquired (or
im m unity” ,
play when the body encounters certain
organisms that overcome the innate
defences. The acquired, or adaptive,
immune system responds specifically
to each type of invading organism, and
retains a memory of the invader so
that defences can be rallied instantly in
any future invasion.
The acquired immune system first
must recognize part of an invading
organism or tumour cell as an antigen
(a protein that is foreign to the body).
One of two types of response (humoral
or cellular) is then mounted against the
Humoral immunity is important in the
defence against bacteria. Following a
complex recognition process, certain
B - l y m p h o c y t e s
vast numbers of antibodies that bind to
the antigens on the invading organism;
phagocytes and destroyed. The binding
of antibody and antigen may activate
creases the efficiency of the phagocytes.
Cellular im m unity is particularly im -
portant in the defence against viruses,
some types of parasites that hide w ith-
in cells, and, possibly, against cancer
cells. Cellular im m unity involves two
These types of white blood cells are found in the
blood and the lymphoid organs (the lymph nodes,
spleen, and thymus). The two main types ( B- and
T-lymphocytes) have different functions but look
similar under the microscope.