I know the following.

  1. Leukocytes (white blood cells) are made in the bone marrow, and naive leukocytes go to the blood vessels. So, leukocytes mainly exist in blood vessels.

  2. Endothelial cells wrapping the capillaries have some gaps, but too narrow for RBCs or leukocytes to leak out (or for bacteria to invade. So, bacteria go to lymphatic vessels).

  3. When leukocytes are needed (like when some foreign molecules invade), leukocytes such as macrophages get out of blood vessels to the tissue of interest by the mechanism of extravasation.

My question is, why there are no RBCs in lymphatic vessels? Is it because they cannot do extravasation, while leukocytes can?

  • 3
    $\begingroup$ That's exactly right $\endgroup$ – De Novo Jul 27 '18 at 3:08

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