Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 3

Immunity, Infection, & Inflammation 67

IMMUNITY

OVERVIEW

Insects and other invertebrates have only
innate immunity.
This
system is triggered by receptors that bind sequences of sugars,
fats, or amino acids in common bacteria and activate various de-
fense mechanisms. The receptors are coded in the germ line, and
their fundamental structure is not modified by exposure to anti-
gen. The activated defenses include, in various species, release of
interferons, phagocytosis, production of antibacterial peptides,
activation of the complement system, and several proteolytic cas-
cades. Even plants release antibacterial peptides in response to in-
fection. In vertebrates, innate immunity is also present, but is
complemented by
adaptive
or
acquired immunity,
a system in
which T and B lymphocytes are activated by very specific anti-
gens. In both innate and acquired immunity, the receptors in-
volved recognize the shape of antigens, not their specific chemical
composition. In acquired immunity, activated B lymphocytes
form clones that produce more antibodies which attack foreign
proteins. After the invasion is repelled, small numbers persist as
memory cells so that a second exposure to the same antigen pro-
vokes a prompt and magnified immune attack. The genetic event
that led to acquired immunity occurred 450 million years ago in
the ancestors of jawed vertebrates and was probably insertion of
a transposon into the genome in a way that made possible the
generation of the immense repertoire of T cell receptors that are
present in the body.

In vertebrates, including humans, innate immunity provides

the first line of defense against infections, but it also triggers

the slower but more specific acquired immune response (Fig-

ure 3–3). In vertebrates, natural and acquired immune mecha-

nisms also attack tumors and tissue transplanted from other

animals.

Once activated, immune cells communicate by means of

cytokines and chemokines. They kill viruses, bacteria, and

other foreign cells by secreting other cytokines and activating

the complement system.

CYTOKINES

Cytokines are hormonelike molecules that act—generally in a

paracrine fashion—to regulate immune responses. They are

secreted not only by lymphocytes and macrophages but by en-

dothelial cells, neurons, glial cells, and other types of cells.

Most of the cytokines were initially named for their actions,

for example, B cell-differentiating factor, B cell-stimulating

factor 2. However, the nomenclature has since been rational-

ized by international agreement to that of the

interleukins.

For example, the name of B cell-differentiating factor was

changed to interleukin-4. A number of cytokines selected for

their biological and clinical relevance are listed in Table 3–2, but

it would be beyond the scope of this text to list all cytokines,

which now number more than 100.

Many of the receptors for cytokines and hematopoietic

growth factors (see above), as well as the receptors for prolactin

FIGURE 3–3
How bacteria, viruses, and tumors trigger innate immunity and initiate the acquired immune response.
Arrows indicate
mediators/cytokines that act on the target cell shown and/or pathways of differentiation. APC, antigen-presenting cell; M, monocyte; N, neutrophil;
T
H
1 and T
H
2, helper T cells type 1 and type 2, respectively.

Naive

T cell

γδT cell

TH 2

B

N M

Plasma

cell

Cytotoxic

lymphocyte

APC

Bacteria

Viruses

Tumors

IL-4

Chemokines

TH 1