The Skin – The skin is obviously a physical barrier to many germs and toxins, as it contains special immune cells called Langerhans cells that act as warning bells to alert the immune system to any foreign agents. Langerhans cells also regulate the immune response to these agents, evident in the skin’s reaction to stinging nettles or a cat scratch. The skin also secretes antibacterial substances which hinder the growth of bugs on our skin.
The Mucus Membrane Linings - The eyes, nose, and mouth are all possible ports of entry for invading germs, but our tears, nasal secretions, and saliva all contain enzymes or cells of the immune system to keep the invaders at bay. The mucus membrane linings of the respiratory, gastrointestinal, and genitourinary tracts also provide one of the first lines of defense against invasion by microbes or parasites.
If the germs make it past this first line of defense, they encounter a number of immune components inside the body including:
- The lymphatic or lymph system
- The thymus gland (in your chest)
- The spleen
- Bone marrow
- White blood cells or leukocytes
The Lymphatic or Lymph System
The swollen glands that we all check for in the neck are in fact lymph nodes that are part of the lymphatic system. The lymph system is similar to the circulatory system, in that it is an interconnected series of vessels carrying lymphatic fluid, except that lymphatic fluid is not pumped around the body (like the heart pumps blood), but rather it moves passively. Fluid oozes in and out of the lymphatic system with normal body and muscle movement. Lymph contains plasma (the watery part of the blood) and helps to carry nutrients, oxygen, and waste products from the blood cells through the capillary walls. Germs generally find their way into this fluid and are then carried to the lymph nodes, which act as filters. The lymph nodes filter the fluid, and if there are any germs, the immune cells in the node rise to the occasion to fight them off. If the lymph nodes swell up during this process, this acts as a sure indication of infection. The filtered lymphatic fluid is then returned to the blood stream where the cycle starts again.
The Thymus Gland
The thymus gland is situated in the chest in front of the heart but behind the breast bone, and is responsible for producing T-cells, one of the important germ-fighting cells of the immune system. The thymus gland is very important for newborn babies (who need it to survive), but as we get older it becomes less important, as other parts of our immune system manage to compensate.
All the cells of the immune system are originally derived from the bone marrow. Our bone marrow produces blood cells – both red cells, which carry oxygen, as well as white blood cells, which are part of the immune system. There are many different types of white blood cells including T-cells, B-cells, natural killer cells, lymphocytes, etc. and they all work together to destroy the foreign cells or germs. The B-cells produce antibodies, or proteins that are specific to the germ (or antigen, which is anything foreign to the body) encountered. Specific B-cells are tuned into specific germs, and when that germ is present, the corresponding B-cell multiplies rapidly and produces the antibodies to destroy that germ. The antibodies then bind to the germ and prevent it from entering our cells. If this is not enough, the antibodies will cover the germ and signal the complement system for assistance.
The spleen is also an important filtration organ, as it searches for and filters out foreign cells as well as old red blood cells that need replacing. In addition, the spleen plays an important role in activating appropriate immune responses by presenting the antigen to the appropriate T or B cells, which in turn can then produce large amounts of anti-bodies.
White blood cells or leukocytes
Immune cells are white blood cells, otherwise known as leukocytes, which are produced in large quantities in the bone marrow. There is a great variety of leukocytes, each with a specific function and role to play in the working of the immune system. Some of these blood cells seek out and destroy foreign organisms, some dispose of infected or mutated body cells, while others release proteins called antibodies that alert other cells to destroy invading organisms.
Antibodies are Y-shaped proteins found in the blood and are made by B-cells. Essentially these proteins are used by the immune system to identify and block the effects of antigens. Thus when an antigen (or foreign cell) is identified, an antibody attaches itself - like a key fits into a lock – and neutralizes the effect of the antigen.
The Complement system
The complement system is a series of different proteins that work with (or compliment) the antibodies. These proteins flow freely in the blood and can therefore rapidly reach the site of an invasion where they can react directly with antigens (molecules that the body recognizes as foreign and potentially dangerous). When triggered, these complement proteins can trigger inflammation, attract eater cells such as macrophages to the area, cover intruders so that eater cells are more likely to destroy them, and directly kill intruders by causing the cells to burst. This in turn signals other ‘clean up’ cells, called phagocytes to come and remove the burst cell. Other substances such as hormones, tumor necrosis factor, and interferons also play an integral part in the functioning of the immune system.