625
CHAPTER SIXTEEN
Lymphatic System and Immunity
16.7
BODY DEFENSES
AGAINST INFECTION
The presence and multiplication of a pathogen in the body
may cause an infection. Pathogens include simple micro-
organisms such as bacteria, complex microorganisms such
as protozoa, and even spores of multicellular organisms such
as fungi. Viruses are pathogens, but they are not considered
Blood capillaries in the red pulp are permeable. Red
blood cells can squeeze through the pores in these capil-
lary walls and enter the venous sinuses. The older, more
fragile red blood cells may rupture during this passage.
Macrophages in the splenic sinuses remove the resulting cel-
lular debris.
Macrophages engulf and destroy foreign particles, such
as bacteria, that may be carried in the blood as it flows
through the splenic sinuses. Lymphocytes of the spleen, like
those of the thymus, lymph nodes, and nodules, also help
defend the body against infections. Thus, the spleen F lters
blood much as the lymph nodes filter lymph.
Table 16.1
summarizes the characteristics of the major organs of the
lymphatic system.
PRACTICE
13
Why are the thymus and spleen considered organs of the
lymphatic system?
14
What are the major functions of the thymus and the spleen?
FIGURE 16.13
Compared to other thoracic organs, the thymus in
the fetus is large, but in the adult is small. Figure is not to scale.
Thymus
in fetus
Thymus
in adult
Spleen
Splenic
artery
Splenic
vein
Artery of pulp
White pulp
Venous sinus
Red pulp
(a)
Capillary
Connective
tissue
Capsule
Capsule
White pulp
Red pulp
(b)
FIGURE 16.14
±Spleen.±(
a
) The spleen resembles a large lymph
node. (
b
) Light micrograph of the spleen (40×).
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