Lymphatic System and Immunity
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-
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.
summarizes the characteristics of the major organs of the
Why are the thymus and spleen considered organs of the
What are the major functions of the thymus and the spleen?
Compared to other thoracic organs, the thymus in
the fetus is large, but in the adult is small. Figure is not to scale.
Artery of pulp
) The spleen resembles a large lymph
) Light micrograph of the spleen (40×).