What roles do lymph nodes and lymph organs play in the immune? 
Exclude the thymus gland.

Hi ccc10-ga!

The lymph nodes and lymph organs are the key elements of the lymphatic
system as described in this article:

"The lymphatic system is one of the most important human body systems.
Every 24 hours, the heart receives approximately 2.83 liters [3
quarts] of lymph that is rich in white blood cells to fight cancer,
viruses, bacteria, and infection."

http://www.pblsh.com/Healthworks/lymphart.html

In order to properly understand what lymph nodes do, we must first
define some other terms:

Lymph - "Lymph [a liquid] is constantly moving around the body. Unlike
the heart-driven circulatory system, the lymphatic system has no
central pump. Lymph circulates by means of the movement of the body's
muscles."

Lymphatics - "The lymphatic system includes a complicated network of
very fine tubes, called lymphatics, that drain lymph from all over the
body. These tubes are comparable in thickness to the diameter of a
needle. The tubes criss-cross the tissues, collecting surplus fluid
from cells."

With the above information we can understand the role of lymph nodes:

Lymph nodes - "At several sites in the human body, the lymphatics join
to form a bundle of tissue called lymph nodes, or popularly known as
lymph glands. The term "lymph gland" technically is not accurate
because "glands" make hormones, and "lymph glands" do not manufacture
hormones.
Any fluid that enters the lymphatic system must pass through at least
one lymph node before advancing to the heart and reentering the
circulartory system.

The individual nodes vary in size from microscopic to approximately
2.5 cm [1 inch] in diameter. Each node consists of a tightly packed
mass of white blood cells forming lymphoid tissue. In the node,
several small lymphatic vessels convey lymph to the node filter and
then to a single larger vessel that carries the fluid out of the node.
Every lymph node has its own small artery and vein.

There are approximately 600 lymph nodes in the human lymph system. The
nodes generally occur in clusters, particularly under the armpits,
behind the ears, in the groin, in the abdominal cavity, behind the
knees, and in other areas.

Lymphoid tissue in the nodes produce antibodies and hold reservoirs of
lymphocytes, as discussed below. Lymph nodes also contain macrophages,
which are large cells that engulf bacteria and other foreign
particles. Thus, lymph nodes function as filters that trap cancer
cells, bacteria, and viruses.

Lymph nodes play an important role in fighting infection, primarily in
their roles in antibody production and lymphocyte storage."

http://www.pblsh.com/Healthworks/lymphart.html

Lymph Organs:

Lymph Organs - "function as larger versions of the lymph nodes as they
provide immunity against pathogenic organisms or other related foreign
particles. There are several lymph organs with various functions
within our body. They include :

Spleen 
Tonsils 
Thymus Gland [details excluded]
Bone Marrow 

Spleen - Purplish-red in color, the spleen is shaped like a loose fist
located in the abdominal cavity. The soft and spongy organ is the
largest of the 4 lymphoid organs and is covered with strong fibers
with the internal section filled with blood. It contains red blood
cells, lymphocytes and macrophages. The spleen has 4 main functions.
They include filtering foreign organism infecting the blood, the
removal of old or damaged platelets and cells, the storing of extra
blood (0.3 gallons) and the formation of certain lymphocytes. If the
spleen is damaged or removed, the individual will be more susceptible
to infections and other complications.

Tonsils - Tonsils are our guards-post as it functions at the easiest
entry for foreign particles into our body – the mouth. The tonsils
contain lymphocytes which will swell in reaction to infection or
exposure to certain stimuli.

Bone Marrow - is the jelly-like substance found inside bone cavities
and inside some of the larger bone. The bone marrow produces red blood
cells, B-lymphocytes (B-cells), monocytes and leukocytes. B-lymphocyte
cells are responsible for the engulfing and ingestion of foreign
particles in the body.

http://library.thinkquest.org/C0115080/?c=lmp_organs
(Note that this link will require you to click a "click here to
proceed link" to view the actual content).

Hope that answers your question, but if you have any problems
understanding the information above please do post a clarification
request and I will respond promptly :)

Cheers!

answerguru-ga

Great overview in response by answerguru.  Couple of comments: 1) Not
all glands create +/- release hormones, only specific types of glands
referred to as "endocrine" glands do that.  So technically, the lymph
"glands", being epithelial in nature and secretory in function, do
qualify as "glands".  Answerguru brilliantly describes the lymphatic
system as a loosely arranged network of tubes that rely mostly on
skeletal muscular contractions as a "pump" to drive lymph back into
the circulatory system.  Recall, however, that by definition the
lymphatic system contains only lymphatic venules, veins, and larger
collecting ducts.  To say that each lymph gland has a proprietary
artery and vein is misleading, as this would classify the glands as
being part of the circulatory system.  This, of course, is what
differentiates "lymph glands" (solely filtering the flow of lymph)
from "lymphoid" or "lymphatic tissues", like the spleen, thymus, and
tonsils (which do have their own arteries/veins and are directly
connected w/the circulatory system, acting as filters for the blood).

...B-lymphocytes are responsible for "tagging" and identifying foreign
invaders within our bodies; they play a large role in activating the
immune system (via communication w/T-lymphocytes, macrophages, and
complement).  The lymphocytes responsible for engulfing and ingesting
would be phagocytes (like macrophages and certain types of
T-lymphocytes/CTKs).

Thanks for your additions sugarfree-ga :)

answerguru-ga

Hi ccc10-ga. Heres a further bit of info on lymphatic problems. I have
been diagnosed with C.L.L. thats CHRONIC LYMPHATIC LEUKEMIA, and was
diagnosed two years ago following a blood test. Unlike anaemia, where
the red cells go down in number and iron in the system is hard to
retain, C.L.L. is an escalation of the number of white blood cells to
extreme amounts. The normal level is in the hundreds but in C.L.L.
they reach thousands. Mine were at 9,000 when diagnosed and went to
ninety thousand before the specialist started treatment. The treatment
is fluoradabine pills which can have side effects. The treatment is
expensive, dont know the exact cost, but my research showed that I
should have started treatment when the reading was thirty thousand, As
the tretament in my case is paid for by the government, for which I am
very thankful,I am sure that the beancounters hope for my early demise
[ to save money]. I investigated the possible cause and found that
duiring my working life i had a job which involved handling the
herbicide 2,4,5,T a component of AGENT ORANGE used in vietnam. A
relative died of liver cancer and he was exposed to the same stuff
while on duty in vietnam.. In my case, placing responsibility is
ongoing. So in a similar vein to what Yul Brynner said about smoking.
Stay away from this stuff. Just don`t SPRAY [SMOKE]