Cells of the Immune System

Only read this page if you are curious about details about how immunity works. It is not necessary to understand this part to make your immunity stronger.

From medical school the mistaken picture I had of the immune system was of a somewhat static thing - somehow made up of compartments of different types of cells and belonging to either cellular or humoral immune functions.

As I prepared this material, I realized just how dynamic the whole of our magnificent immune world is. Most important is that our immune system must recognize what is 'us' - what rightly belongs to the healthy tissues of our body and therefore not attack it. It must also differentiate what we can healthily accept into our body, i.e. foods, a fetus which is half not us, or symbiotic bacteria (those living in peaceful co-existence with us) and which peacefully co-exist in many of our body spaces and which function to ward off infections, and what we must reject in order to stay healthy.

In this wonderful dance, the cells of the immune system are constantly communicating with each other and with other regulatory systems of the body. In some cases, they produce chemical messages to tone down their attack responses and reduce inflammation, in others, the messages turn on the heat and literally produce a fever.

In this extraordinary intelligence and communication, I am reminded of ants as they forage for food, the incoming ants touching the antennae of those going out, mysteriously informing them where to go.

Cells of the Innate Immune System:

Natural Killer Cell Lymphocytes

Relatively rare (5 - 10% of circulating lymphocytes), killer T cells come from both bone marrow and the thymus. The thymus, located just behind the breastbone, is like the starting point for the memory immune cells. Natural killer cells are able to attack tumor cells and pathogens which don't yet have antibodies against them. This makes Killer T cells very important because they are unique first line defenders. Some Killer T cells are able to mop up dead red blood cells. Others are able to stimulate bone marrow stem cells. Stem cells are a very exciting area of research because they may be able to repair damaged tissues.

There is some evidence that AFA, a blue green algae, may be able to stimulate stem cells and aid in regeneration of damaged tissues.

 

Monocytes / Macrophages

Monocytes arise from within the bone marrow and circulate with a half-life of 1 - 3 days. They leave the blood by first sticking to the blood vessel walls and then squeezing between the cells of the vessel where they then transform into macrophages. They are mainly found in lymph tissue i.e. in the spleen but, in essence, everywhere there is a need to defend against pathogens. So the lungs and especially the lymph tissue in the intestine are areas of high numbers of macrophages. They are part of the first line of defense. They destroy antibody coated bacteria, tumor cells and virus infected cells.

New microscopic techniques have shown macrophages shooting out a "pod" to engulf a problematic bacteria. This is a very dynamic event!

Sometimes viruses invade macrophages, triggering cell death. These dying cells are recognized by dendritic cells, prompting a process of antibody production which, once created, will assist in killing the same type of virus.

Macrophages are activated by beta-glucan, making them even more effective in fighting invaders and strengthening our immunity.

Dendritic cells

Originating in the bone marrow, these versatile starfish-like cells are powerful interceptors of incoming bacteria and viruses. They do circulate in the blood but end up either in tissues through the body or in pockets in lymph glands where they trap pathogens. They then pass the trapped and partially digested pathogens to lymphocytes. This process allows lymphocytes to produce unique antibodies against the pathogen. Once the antibody is in the system, a much more aggressive response is available to fight the pathogen next time we encounter it.

Eosinophils

The cells are identified by the presence of granules inside their cells. The chemicals they contain play a role in amplifying inflammation. Unchecked, they are able to cause an attack of the tissues of the body. Their toxic contents include a variety of killing substances including microbial superoxide radicals. While extremely effective in killing pathogens, healthy tissues can be damaged by their release.

Basophils

Basophils are white cells capable of releasing cytokines, substances which open up the blood vessel walls. This sort of 'leakage' is the type of reaction seen in acute allergic states associated with swollen lips, eyelids and difficulty breathing (anaphylaxis).

Tissue Mast Cells

Like basophils, mast cells are able to release tissue necrosis factors, interlukins and cytokines in response to bacterial invasion. These substances promote a killing response to the invader and prime the lymphocytes to begin producing antibodies.

Epithelial Cells

These important cells originate in the tissues of their respective organs. They are the skin of our body - a one cell thick layer of cells which separate the outside from the inside of our body.

In the lung, epithelial cells, illustrated in the image to the right, produce surfactant, a soap-like protein which lowers surface tension in the tiny air sacs, alveoli. Without surfactant, the alveoli would collapse. Another function of surfactant is to trap microbes and promote their clearance from the lung. Respiratory epithelium also includes specialized cells which sprout cilae, brush-like structures which sweep foreign material upward from our lungs.

In the intestine epithelial cells play a vital role as the one cell thick layer responsible for nutrient absorption. They are tightly bound to their next door neighbor (tight junction). They physically resist invasion by microbes.

Cells of Acquired Immunity

Acquired, or adaptive immunity is tied to specific responses to antigens from foreign antigens or pathogens.

'B' Lymphocytes and 'T' Lymphocytes

The image to the right is of a circulating lymphocyte surrounded be red blood cells.

'Bursal' lymphocytes are found clustered in the core of lymph tissue scattered throughout the body. They share a common origin with their co-workers, the 'T' (thymus) lymphocytes. Both originate from a common line of stem cells in the bone marrow. After maturation in the bone marrow, they enter circulation and home in on lymph tissue through the body; Peyer's patches in the intestine, in the respiratory tract, and spleen as well as in the skin. There, they await activation by cells of the innate immune system; antigen presenting cells, dendritic cells, macrophages etc.

T Lymphocytes

New T lymphocytes are created in the thymus and throughout the peripheral lymph tissue. THey make up about 75% of the blood lymphocytes. In the tissues, they cluster around the outer edges of lymph glands where they have two very important functions.

1. by regulating immune responses. They keep a 'finger on the pulse' of the immune system. This 'sense' lets the other cells know how aggressively to respond.

2.they receive incoming antigen from the antigen presenting cells, then create chemical-genetic messages which they pass to B lymphocytes. The B lymphocytes then create antibodies.

3. In the case of TB and some types of bacteria, they are able to mount a direct attack against the invaders.

B Lymphocytes

Mature B lymphocytes make up about 15% of the blood lymphocytes and about half of those in the spleen. They are constantly being created in the bone marrow. Their primary job is to produce antibodies, specific proteins designed to attach to corresponding sites on microbes.

Usually these activation signals pass through the genetic apparatus of T lymphocytes. Following entry onto the B lymphocytes, proteins are created which give rise to antibodies. These antibodies are unique to each type of pathogen. It has been estimated that we carry trillions of individual antibodies. These antibodies then lie in wait for their particular microbe or other foreign material.

B lymphocytes are also able to directly respond to 'native' (unprocessed) antigen. In this role, they can create new antibodies in direct response to microbial invasion. In this situation, they can also feedback information to T lymphocytes.

Chemical Messengers of the Immune System

In order to regulate immunity, our cells create a number of chemical messengers. Some are proteins and get passed directly from cell to cell. Some are responsible for increasing the inflammatory response and some for decreasing it.

Complement

A wide ranging group of soluble proteins and enzymes which target pathogens and link themselves to the pathogen. Thus linked, the pathogen can be destroyed by the cells of the innate immune system, neutrophils and macrophages.

Cytokines

Proteins which are able to be dissolved and which regulate the growth and activity of immune cells. They also regulate the amount of inflammation in health and when there is an illness. Types of cytokines include interlukins, interferons, tumor necrosis factor, platelet growth and platelet activating factors.

Antibodies

Small proteins made primarily by "B" lymphocytes ( white blood cells produced in bursal lymph tissue). Antibodies are created after the innate immune system meets a "foreign" microbe or substance. Once they begin circulating, they are ready-made traps for that microbe. They attach to the invader and help lead it to its destruction.

Immunoglobulins

These proteins are composed primarily of antibodies and fall into several classes:

In innate immunity, Immunoglobulin A (IgA) is the most important of these. It is secreted into mucus which lines the body's hollow tracts; respiratory, digestive, urinary, reproductive and eye. These antibodies are an important first line of defense.