Page 355 - Veterinary Immunology, 10th Edition

cells. They have a cortex containing germinal centers and B cells. T
VetBooks.ir cells predominate at the center in association with lymphatic

sinuses. These cells differ, however, from those found in
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conventional lymph nodes (more γ/δ , WC1 T cells, fewer CD8 T
cells) (Chapter 14). Intravenously injected carbon particles are
trapped in the sinusoids of hemolymph nodes, suggesting that they
may combine features of both the spleen and lymph nodes.

Spleen

Just as lymph nodes filter antigens from lymph, so the spleen filters
blood-borne pathogens and antigens. Indeed, the spleen can be
considered a specialized lymph node for blood-borne antigens. The
filtering process removes antigenic particles such as blood-borne

microorganisms, cellular debris, and aged blood cells. This filtering
function, together with highly organized lymphoid tissue, makes
the spleen an important component of the immune system. In

addition to its immune functions, the spleen also stores red cells
and platelets, recycles iron, and undertakes red cell production in
the fetus. As a result, the spleen consists of two forms of tissue. One
is used predominantly for blood filtering and for red cell storage,
called the red pulp. It contains large numbers of antigen-presenting

cells, lymphocytes, and plasma cells. Macrophages in the red pulp
specialize in removing aged red blood cells and so regulate iron
recycling. The other tissue is rich in both B and T cells where

immune responses occur and is called the white pulp. The white
pulp is separated from the red pulp by a region called the marginal
zone. This zone contains numerous macrophages and dendritic
cells as well as a large population of B cells. The spleen is not
supplied with lymphatic fluid, although it does possess efferent

lymphatics.

Structure of White Pulp

Arteries entering the spleen pass through muscular trabeculae
before entering the white pulp and branching into arterioles.

Immediately on leaving the trabeculae, each arteriole is surrounded
by a layer of lymphoid tissue called the periarteriolar lymphoid
sheath (Fig. 12.19). The arteriole eventually leaves this sheath and

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