Lymph Nodes -- The lymph nodes function as an immunologic filter for
the bodily fluid known as lymph. Lymph nodes are found throughout
the body. Composed mostly of T cells, B cells, dendritic cells and
macrophages, the nodes drain fluid from most of our tissues.
Antigens are filtered out of the lymph in the lymph node before
returning the lymph to the circulation. In a similar fashion as the
spleen, the macrophages and dendritic cells that capture antigens
present these foreign materials to T and B cells, consequently
initiating an immune response

Lymphatic vessels and lymph nodes are the parts of the special
circulatory system that carries lymph, a transparent fluid
containing white blood cells, chiefly lymphocytes.


Lymph bathes the tissues of the body, and the lymphatic vessels
collect and move it eventually back into the blood circulation.
Lymph nodes dot the network of lymphatic vessels and provide meeting
grounds for the immune system cells that defend against invaders.
The spleen, at the upper left of the abdomen, is also a staging
ground and a place where immune system cells confront foreign
microbes.

Pockets of lymphoid tissue are in many other locations throughout
the body, such as the bone marrow and thymus. Tonsils, adenoids,
Peyer's patches, and the appendix are also lymphoid tissues.

Both immune cells and foreign molecules enter the lymph nodes via
blood vessels or lymphatic vessels. All immune cells exit the
lymphatic system and eventually return to the bloodstream. Once in
the bloodstream, lymphocytes are transported to tissues throughout
the body, where they act as sentries on the lookout for foreign
antigens.






Bone Marrow -- All the cells of the immune system are initially
derived from the bone marrow. They form through a process called
hematopoiesis. During hematopoiesis, bone marrow-derived stem cells
differentiate into either mature cells of the immune system or into
precursors of cells that migrate out of the bone marrow to continue
their maturation elsewhere. The bone marrow produces B cells,
natural killer cells, granulocytes and immature thymocytes, in
addition to red blood cells and platelets.

How the Immune System Works
Cells that will grow into the many types of more specialized cells
that circulate throughout the immune system are produced in the bone
marrow. This nutrient-rich, spongy tissue is found in the center
shafts of certain long, flat bones of the body, such as the bones of
the pelvis. The cells most relevant for understanding vaccines are
the lymphocytes, numbering close to one trillion.

The two major classes of lymphocytes are B cells, which grow to
maturity in the bone marrow, and T cells, which mature in the
thymus, high in the chest behind the breastbone.

B cells produce antibodies that circulate in the blood and lymph
streams and attach to foreign antigens to mark them for destruction
by other immune cells.

B cells are part of what is known as antibody-mediated or humoral
immunity, so called because the antibodies circulate in blood and
lymph, which the ancient Greeks called, the body's "humors."

Certain T cells, which also patrol the blood and lymph for foreign
invaders, can do more than mark the antigens; they attack and
destroy diseased cells they recognize as foreign. T lymphocytes are
responsible for cell-mediated immunity (or cellular immunity). T
cells also orchestrate, regulate and coordinate the overall immune
response. T cells depend on unique cell surface molecules called the
major histocompatibility complex (MHC) to help them recognize
antigen fragments.