Human Physiology, 14th edition (2016)

498 Chapter 15

Figure 15.3 Life cycle of the human immuno-
deficiency virus (HIV). This virus contains RNA rather than DNA.
After the viral RNA is inserted into the cell, it (1) is transcribed by
reverse transcriptase into complementary DNA (cDNA), which
(2) enters the nucleus and integrates into the host DNA. The cDNA
then directs the synthesis of RNA, which (3) codes for viral proteins,
enabling the assembly of new virus particles. (4) These new viruses
are then budded from the host cell, so they can infect other cells.

Transcription

Reverse transcriptase

Viral RNA

Reverse

transcription

cDNA

Viral protein synthesis

Cytoplasm

Host cell

HIV

Assembly

Budding

1

2

3

4

Nucleus

Host DNA

Integration

Stimulation Inhibition

Macrophage phagocytosis Cell division

Activity of cytotoxic (“killer”) T cells Tumor growth

Activity of natural killer cells Maturation of adipose cells

Production of antibodies Maturation of erythrocytes

Table 15.3 | Effects of Interferons

produced. Most antigens are large molecules (such as proteins)

with a molecular weight greater than about 10,000, although

there are important exceptions. Also, most antigens are foreign

to the blood and other body fluids. This is because the immune

system can distinguish its own “self ” molecules from those of

any other organism (“nonself ”) and normally mounts an immune

response only against nonself antigens. The ability of a molecule

to function as an antigen depends not only on its size but also

on the complexity of its structure. The plastics used in artificial

implants are composed of large molecules, but they are not very

antigenic because of their simple, repeating structures.

A large, complex molecule can have a number of different

antigenic determinant sites (also called epitopes ), which are

areas of the molecule that stimulate production of, and combine

with, different antibodies. Most naturally occurring antigens

have many antigenic determinant sites and stimulate the pro-

duction of different antibodies with specificities for these sites.

Haptens

Many small organic molecules are not antigenic by themselves but

can become antigens if they bind to proteins (and thus become anti-

genic determinant sites on the proteins). This discovery was made

by Karl Landsteiner, who also discovered the ABO blood groups

(chapter 13, section 13.2). By bonding these small molecules—

which Landsteiner called haptens —to proteins in the laboratory,

new antigens could be created for research or diagnostic purposes.

The bonding of foreign haptens to a person’s own proteins can

also occur in the body. By this means, derivatives of penicillin, for

example, that would otherwise be harmless can produce fatal aller-

gic reactions in susceptible people.

Immunoassays

When molecules act as antigens and can bind to antibodies, the

antigens can be assayed (detected and measured) by means of

the antigen-antibody reaction. If the antibodies are attached to

the surface of cells, or to artificial particles such as small poly-

styrene beads (in commercial diagnostic tests), the antigen-

antibody reaction becomes visible because the cells or particles

agglutinate (clump). The agglutination is caused by the antigen-

antibody bonds, which create bridges between the cells or par-

ticles ( fig.  15.4 ). These agglutinated particles can be used to

assay a variety of antigens, and tests that utilize this procedure

are called immunoassays. Blood typing and modern pregnancy

tests are examples of such immunoassays. In order to increase

their sensitivity, modern immunoassays generally use antibodies

that exhibit specificity for just one antigenic determinant site.