The immune system protects the body from invaders, such as viruses and bacteria. The system includes the lymphatic system and various white blood cells (WBCs).
The lymphatic system has a network of
vessels that drain intracellular fluid from the intracellular space and return
it to the blood. The lymph vessels are not connected to the heart and do not benefit
from its contraction. They, like many veins, are embedded in skeletal muscle and rely on muscle contractions to move
fluid (intracellular fluid inside lymph
vessels). Also like veins, lymph vessels contain valves that prevent backflow. The
lymph vessels deliver the fluid into large
veins inthe chest, where the fluid again becomes part of the blood.
Connected to lymph vessels are
small masses of spongy tissue called lymph
nodes, which remove contaminants such as bacteria and dead cells from lymphatic
fluid. In addition, the nodes are homes for certain types of white blood cells (WBCs).
White blood cells
A healthy human typically has 5,000 to 9,000 WBCs per milliliter of blood. When
bacteria or other foreign particles are present, however, WBCs rapidly proliferate. Unlike RBCs (red blood cells), WBCs can
move on their own; they frequently pass through the
blood vessels and go into intercellular space and the lymphatic system in search of
Most WBCs are granulocytes.
These are phagocytes
("eating cells"), moving like amoebas to surround
and engulf foreign particles. Monocytes, the smallest group of WBCs, move out of
blood and into intercellular space whenever an infection develops. There they turn into phagocytic
macrophages and destroy invaders. Monocytes also destroy worn-out RBCs.
Lymphocytes are WBCs more common
in the lymph system than in the blood. There are
two classes: T cells and B cells. Helper T
cells and suppressor T cells regulate the immune response, including the
activities of B cells. Killer T cells kill cells they attack,
such as body cells infected with viruses. B cells
produce antibodies, highly specific proteins secreted in response to foreign chemicals called
such as protein molecules on the surface of bacteria. The antibodies bind to and inactivate the antigens. Some B
cells, called memory cells, have learned to recognize a specific invader and patrol the body to defend against it. If the
invader reappears, the memory cells trigger a massive defense. This reaction protects a person from getting
certain diseases more than once, and is the basis of vaccination against
disease. (The New York Times Guide to Essential Knowledge)