"...look into all things with a searching eye” - Baha'u'llah (Prophet Founder of the Baha'i Faith)


Dec 15, 2012

Our Immune System

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).

Lymphatic system

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 the lymphatic 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 walls of blood vessels and go into intercellular space and the lymphatic system in search of invaders.

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 antigens, 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)