The immune cells that make up our immune system consists of several different types of white blood cells or leukocytes that work together to keep us healthy. They circulate through the body between the organs and nodes via the lymphatic system. Most however are stored in the lymphoid organs until needed.
There are two types of leukocytes :
- the phagocytes
– cells that chew up invading micoorganisms and
- the lymphocytes
– these immune cells have receptors that remember and recognise previous invaders. B lymphocytes cell (B cell) and the T lymphocyte cell (T cell) are the main types of lymphocytes.
A brief description of some of the most important immune cells are highlighted here.
Phagocytes or scavenging eater cells comprises both monocytes and granulocytes which fight bacteria primarily. Other types of phagocytes make sure that the body responds appropriately to a specific type of invader. They’re known as a form or non-specific immunity as they don’t discriminate the invaders.
Neutrophils are the main white blood cells that circulate in the blood stream. They are also granulocytes which contain granules filled with potent chemicals that play a key role in acute inflammatory reactions.
When you have a bacterial infection, the level of neutrophils are elevated. Like NK cells (see below) they produce toxins that kill the antigen and then ingest it. However NK cells are autonomous but neutrophils are summoned into action by other immune cells.
Eosinophils is another granulocyte which carry receptors for IgE and will destroy IgE coated antigens. Their secretions inactivate and destroy cancer cells.
Basophils are also filled with granules of toxic chemicals that can digest microorganisms. They are responsible for the symptoms of allergy. They enter tissues only when they are summoned to the inflamed sites.
Monocytes and Macrophages
Monocytes, which can ingest dead and damaged cells, migrate into tissues and develop into macrophages.
Macrophages are large immune cells residing in organs that directly interface with the bloodstream or come into contact with the outside world like the liver and lung. They do circulate in the bloodstream but are stationed at the respective organs.
Their function is similar to neutrophils in that they devour foreign material and worn-out cells and present them to various T cells. Although they have their own positions to protect however, when a part of the body becomes infected they rush to the infected area helping the neutrophils to “munch up” (phagocytosis) the antigens.
Macrophages also release a substance called pyrogen which signals the body to raise its temperature to “cook” the virus or bacteria.
B Cells, produced by the bone marrow, spleen and lymph nodes, are critical for normal immune function. They are called B cells because they mature in the bone marrow.
Their main role is to produce antibodies also known as immunoglobulins, that circulates in the blood and lymph.
Antibodies ambush antigens in the bloodstream. There are many types of antibodies produced, each specific to bind one type of antigen, ie make one type of antibody. Not all of the immune cells do the same job. Some immune cells attack antigens ie foreign proteins (like pollutants, toxins, microorganisms…) at random.
If any antigen escape the first line of defense, they will be captured by the B cells. What the B cells does is, they “tag” the antigens with antibodies which alert other immune cells of danger ahead. This simple action neutralise the antigen and render them harmless.
B cells have good memory. They remember what antibodies they have produced against which antigen, ie they have created immunity against this antigen. Should the same antigen attack again, it will be quickly destroyed before it can do any harm.
Vaccination makes use of this unique quality of B cells. The vaccine introduced into our bodies stimulate the B cells to build antibodies and be ready for the real onslaught of the disease eg smallpox, tetanus, whooping cough, chicken pox and polio.
B cells does not destroy/penetrate cells of the antigen, it only tags (coats) them as a signal for other cells to do their job. These tagged antigens are destroyed by T cells.
T Cells (thymus-derived lymphocytes)
T cells, which mature in the thymus, work in tantem with B cells. They do not produce antibodies but have surface receptors related to the Igs. Not all T cells are the same. Some attack infected or cancerous cells while others look for the antibody tag before attacking a specific invader.
- Cytotoxic T cells or killer cells response to specific foreign antigen, race to the site of invasion and attach themselves to the invaders and destroy them with destructive chemicals they produce. They are especially useful in attacking cancer cells and viruses as they can see the small fragments peeking out of the cell membrane where it is hiding while they (virus) grow inside the infected cell.
- Helper T cells (also known as T4 or CD4 cells) coordinate immune responses. Some will stimulate B cells to produce antibodies, others call in the phagocytes to gobble the microbes and still others activate other T cells.
- Suppressor T cells tells the B cells to stop producing when there’s enough antibodies and the cytotoxic T cells to retreat.
T cell receptors don’t recognise some antigens until they are processed ie engulfed by the antigen-presenting cell (APC), a specialised white blood cell.
This APC breaks the antigen down to smaller fragments which are then linked to special marker cells collectively called major histocompatibility complex (MHC).
The T receptor can now recognise the antigen and will bind to it releasing lethal substances that perforate the cell membrane and kill the cell.
The negative side of these MCH is seen in the transplant of organs. This new organ is covered with MCH that is not your own so the T cells will try and destroy it.
Natural Killer Cells
Natural Killer cells or NK cells for short, the aggressive T cells have many different functions. They are a type of lymphocytes and have the innate ability to detect and attack an intruder on its own, unlike the cytotoxic T cells which only attack those that covered with MCH. Therefore they primarily deal with controlling cancer or tumor cells and acute infection.
NK cells produce a substance called interferon which prevents viruses from replicating. They also release poisonous chemicals like nitric oxide that will destroy the internal workings of the invader. NK Activity is an excellent barometer of overall health and can be measured by a simple blood test.
People with low NK cell activity often have a high incidence of cancer. Studies have shown that people with high NK activity doesn’t fall sick easily and vice versa. You can boost your NK activity by taking supplements.
Types of Antibodies
There are 5 groups of antibodies : IgA, IgG, IgM, IgE and IgD. (Ig stands from Immunoglobulin) They have different roles to play.
- IgA is the primary antibody that is released in tears, saliva, the walls of the mucus membranes and the cells that line the intestinal tract, guarding the entrances to the body. A healty person produces upto 10 gm of IgA per day. Colostrum from mother’s milk contains a very high amount of IgA. Colostrum is therefore very important to newborns as this cannot pass through the placenta.
- IgM is produced after the initial exposure to an antigen eg after receiving a vaccination, the IgM level will be increased. It is very effective in killing bacteria.
- IgG is the most dominant/common type of antibody. It works efficiently to coat microbes, thus attracts the other immune cells quickly.
- IgE is the antibody involved in allergic reactions. It helps the body to reject substances such as dust and pollen. IgE stimulates special cells called mast cells to release histamine, a chemical that is important for digestion and the dilation of small blodd vessels. However excess IgE can trigger allergic reaction.
- IgD is found in small amounts circulating in the blood but it’s function is not fully understood. It is believed that they remain attached to B cells and plays a key role in initiating early B cell response.
Immune cells communicate with each other
Organs of the immune system communicate through cytokines, a protein secreted by the cells, which transmit information to other immune cells. For example, macrophages summon neutrophils via cytokines; NK cells mobilise T cells by sending out cytokines. Cytokines are central in activating many other immunological cells.
A particular type of cytokine called interleukins, talk to nerve cells, thereby creating a link between the immune system and the nervous system. Also since all immune cells begin as immature stem cells in the bone marrow, they respond to different cytokines to grow into specific immune cells such as T cell etc.
Because stem cells are capable of growing into specific type of cells, researchers are now investigating the possibility of using them to regenerate damaged immune response in some immune system disorders.