CK12 Life Science

(Marvins-Underground-K-12) #1
Figure 21.25: ( 18 )

also contract to help move the lymph along. The lymph finally reaches the main lymph
vessels in the chest. Here, the lymph drains into two large veins. This is how the lymph
returns to the bloodstream.


Before lymph reaches the bloodstream, pathogens are removed from it at lymph nodes.
Lymph nodesare small, oval structures located along the lymph vessels. They act like
filters. Anypathogensfilteredoutofthelymphatlymphnodesaredestroyedbylymphocytes
in the nodes.


Lymphocytes


Lymphocytes are the key cells of an immune response. A photograph of a lymphocyte is
shown inFigure21.26. The lymphocyte shown in the figure is greatly magnified. There are
trillions of lymphocytes in the human body. They make up about one quarter of all white
blood cells. Usually, fewer than half of the body’s lymphocytes are in the blood. The rest
are in the lymph, lymph nodes, and lymph organs.


There are two main types of lymphocytes: B cells and T cells. Both types of lymphocytes
are produced in the red bone marrow. They are named for the sites where they mature. The
Bin B cells stands for “bone.” B cells mature in red bone marrow. TheTin T cells stands
for “thymus.” T cells mature in the thymus gland. B and T cells must be “switched on” in
order to fight a specific pathogen. Once this happens, they multiply and produce an army
of cells ready to fight that particular pathogen.


HowcanBandTcellsrecognizespecificpathogens? Pathogenshaveproteinsthatareforeign

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