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Red bone marrow

Key:

= Site of lymphocyte origin

= Site of development of immunocompetence as B or T cells; primary lymphoid organs = Site of antigen challenge,

Circulation in blood

1

Immature lymphocytes

1

activation, and final diff erentiation of B and T cells

Bone marrow

1 Lymphocytes destined to become T cells migrate to the thymus and develop immunocompetence there. B cells develop immunocompetence in red bone marrow.

2

2 After leaving the thymus or bone marrow as naïve immunocompetent cells, lymphocytes “seed” the lymph nodes, spleen, and other lymphoid tissues where the antigen challenge occurs.

3

3 Antigen-activated immunocompetent lymphocytes circulate continuously in the bloodstream and lymph and throughout the lymphoid organs of the body.

Thymus

3

Activated Immunocompetent B and T cells recirculate in blood and lymph

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings

Figure 21.8

2

Immunocompetent, but still naive, lymphocyte migrates via blood

Lymph nodes, spleen, and other lymphoid tissues

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