The human body has natural protection which is called the immune system. This acts to prevent or exclude harm by pathogens.
The human body is often described as being 'at war'. By this, it is meant that the body is constantly under attack from things that are trying to do it harm. These include toxins, bacteria, fungi, parasites and viruses. All of these can, under the right conditions, cause damage and destruction to parts of the body and if these were left unchecked, the human body would not be able to function. It is the purpose of the immune system to act as the body's own army, in defence against this constant stream of possible infections and toxins. The human immune system is divided into two broad groups called the acquaried immune system and the innate immune system.
The innate immune system is always working to protect the body and does not require any special preparation to stop infection. The acquired immune system needs to be 'primed' before it can work to its full effectiveness though, and is only really effective after it has seen a possible infective agent before.
An overview of these different systems is given in the chart over the page.
Structure and organs of the immune system
The lymphatic system
The lymphatic system is almost equivalent to the blood vessels, only instead of carrying blood through the body, the lymphatic system carries a substance called 'lymph'. Lymph is excess tissue fluid that has been drained from the body compartments. Lymphatic fluid is usually clear, watery, and has the same constitution as the blood, but without any cells. The lymphatic system is a complex network of lymphatic vessels (that carry the lymph), along which there are occasional lymph nodes. After the lymphatic system has collected all the lymph, this passes through the lymph nodes before being put back into the blood via a large vein just below the neck. In the lymphatic system there are lots of cells called lymphocytes (the T and B cells) that circulate around and are part of the acquired immune system.
Lymphoid tissue is scattered throughout the body and is home to the lymphocytes. Lymphocytes are packed into clusters in the walls of parts of the body that are often exposed to foreign substances. These sites include the gastrointestinal system as well as the tonsils which play a role in protecting the body from any air-borne infections.
Lymph nodes are small, oval structures that can be anywhere from 1mm to 25mm big. Blood vessels and nerves attach to the lymph nodes, as well as two sets of lymphatic vessels - those that enter the lymph node and those that leave it.
The lymph enters from one side and slowly moves past all the cells of the lymph node before leaving through the other lymphatic vessel. This allows the lymph time to access as many of the lymphatic cells as possible. In the lymph node there is a dense packaging of immune cells such as macrophages. These are the 'big eaters' and will engulf and destroy anything dangerous that they can. They also play a role in showing these substances to the T and B cells (which is described in more detail under the Acquired Immune System). There are also areas of the lymph node called 'germinal centres' where all the b cells multiply to fight off infection. In another part of the lymph node, there are mostly T cells. When they need to, the lymphocytes leave the lymph node and enter the circulation to fight infection.
The lymph nodes are there as a filter for the lymph before it re-enters the venous system. 99% of all the foreign substances that arrive at the lymph node are removed. Lymph nodes are found in regions such as the base of the neck, the armpit and the groin. Swelling or inflammation of these nodes is usually in response to an