Human Physiology, 14th edition (2016)

The Immune System 503

A lymphocyte circulates throughout the body, going from one

secondary lymphoid organ to another until it encounters the anti-

gen that is specific to its receptors. Exposure of a B lymphocyte

to the appropriate antigen activates the B cell and causes it to

enter a germinal center of a secondary lymphoid organ, where

it undergoes many cell divisions. Some of the progeny become

memory cells; these are visually indistinguishable from the orig-

inal cell and are important in active immunity. Others are trans-

formed into plasma cells ( fig.  15.7 ). Plasma cells are protein

factories that produce about 2,000 antibody proteins per second.

The antibodies that are produced by plasma cells when B

lymphocytes are exposed to a particular antigen react specifically

with that antigen. Such antigens may be isolated molecules, as

illustrated in figure 15.7 , or they may be molecules at the surface

of an invading foreign cell ( fig.  15.8 ). The specific bonding of

antibodies to antigens serves to identify the enemy and to activate

defense mechanisms that lead to the invader’s destruction.

15.2 FUNCTIONS

OF B LYMPHOCYTES

B lymphocytes secrete antibodies that bind to specific

antigens. This bonding stimulates a cascade of reactions

whereby a system of plasma proteins called complement is

activated. Some of the activated complement proteins kill

the cells containing the antigen; others promote phagocyto-

sis, resulting in a more effective defense against pathogens.

| CHECKPOINT

1a. List the phagocytic cells found in blood and lymph,

and indicate which organs contain fixed phagocytes.

1b. Describe the actions of interferons.

2a. Distinguish between innate and adaptive immunity,

and describe the properties of antigens.

2b. Distinguish between B and T lymphocytes in terms of

their origins and immune functions.

2c. Identify the primary and secondary lymphoid organs

and describe their functions.

3. Describe the events that occur during a local inflammation.

LEARNING OUTCOMES

After studying this section, you should be able to:

4. Describe B lymphocytes and antibodies and explain
   how they function.


5. Describe the complement system and explain its
   functions.

Figure 15.7 B lymphocytes are
stimulated to become plasma cells
and memory cells. B lymphocytes have
antibodies on their surface that function
as receptors for specific antigens. The
interaction of antigens and antibodies on
the surface stimulates cell division and the
maturation of the B cell progeny into memory
cells and plasma cells. Plasma cells produce
and secrete large amounts of the antibody.
(Note the extensive rough endoplasmic
reticulum in these cells.)

1. Antigens bind
   to antibody
   receptors


2. Proliferation
   (mitosis) causes
   formation of
   a clone


3. Plasma cells and
   memory cells are formed

+ Antigen

Antibody

Mitochondrion

Memory cell Memory cell

Plasma cell Plasma cell

Endoplasmic

reticulum

Nucleus