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[I157]Immune Response To Infection

by K Bakhru, K B

The first phase of the immune response occurs when antigens enter the body, usually through the mouth, nose or skin. When this happens, inflammation occurs in the body and the immune system is stimulated to produce more white cells. The task of these extra white cells is to eliminate the offending antigen. If this is successful, the inflammation will subside.

The same process will apply when the body is stressed emotionally or physically. Further stress on the immune system can be caused by an allergic reaction due to the presence in the body of antigens, in the form of allergenic substances.

The second phase of the immune response takes place when the white cells (called leucocytes) are unable to destroy the antigen. The immune system then commences the formation of antibodies, called immunoglobulin, which are tailor-made to combat the antigen and destroy it. Once antibodies have been created to fight a specific antigen, they remain dormant in the body, ready for any future invasion by the antigen. The body now has the capacity to produce more of that antibody, faster, the next time invasion takes place. This is the principle used in immunization and is called Specific Immunity.

Thus, there are two stages to an immune response. The first being, the production of white cells, and the second being, the creation of specific antibodies. Normally, the white cells and the antibodies destroy the antigen and the body makes a speedy recovery. Sometimes however, this does not happen and the result is an allergic reaction which the immune system cannot deal with.

The Allergic Reaction:

There are five different classes of immunoglobulins in the body. These are known as IgG, IgA, IgM, IgD and 19B. IgG is the principal immunoglobulin in the blood and internal fluids. Its job is to remove soluble antigens from the body, in conjunction with other immune complexes. IgA, and to a lesser extent IgM are the main secretory immunoglobulins. They form a protective coating on the body's mucosa, thereby limiting entrance of antigens through the mucosa surfaces, such as in the nasal passages and the gut. Evidently, the physiological functions of IgD and IgE are not fully understood. It is thought that IgE may play an important part in ridding the body of mucosal infection but is ineffective in ecological illness. On the other hand, IgD may act as the trigger to initiate immune responses as it has been observed that, when a mucosal surface is under attack by antigens, initiation of IgE production is dependent upon a lymphocyte that contains IgD.

When the antigen is an allergenic substance (the allergen), the leucocytes and immunoglobulins are unable to cope with it. The antibody and the antigen react, causing a malfunction in the body's defences. The mast cells, which are found in mucous membrane and connective tissue, break up. As a result, chemicals such as histamine, are released and these cause irritation and damage.

Some antigens may reach the bloodstream by way of the body's mucous surfaces. There, they attach themselves to red and white cells or form immune complexes with specific antibodies. These are carried around the body and can cause direct tissue injury, for example a precipitate in connective tissue which can block small blood vessels. This results in fever, aching, muscle pains, and can happen after eating a certain food. Those mysterious, but troublesome, back and joint pains which appear to have no rational cause can be the result of an allergic reaction. Alternatively, the symptoms may not relate to a specific 'target' area. Instead, inflammation and fever may occur over the entire body. This can make it difficult to identify as an allergic reaction.

When inflammation or fever occurs as part of the immune response, it may be localized at an area of infection or be dispersed throughout the entire body. If the latter happens, the body temperature will increase and cause greater enzyme activity. This in turn, increases the metabolic rate; providing extra energy for the production of the leucocytes and immunoglobulins needed to fight the invading antigens. Although, in the case of an allergic reaction, this process is not successful, it may explain why allergy sufferers, particularly of food and chemical allergies, often tend to feel overheated.

Several approaches have been taken in an attempt to determine whether Vitamin A supplementation enhances immune response and resistance or recovery from infection. In some investigations, researchers have attempted to correlate plasma concentrations of beta-carotene or retinol with immune response or susceptibility to infection. One limitation of this approach is related to the fact that plasma concentrations may have depressed plasma retinol levels as a result of disease. Therefore, it is not possible to establish whether low plasma retinol levels resulted in suppressed immune response or if plasma retinol levels decreased in response to disease or infection. Another approach used is to supplement the diet with retinol precursors and examine immune response at a later time point. This approach may be useful in examining the particular aspects of immunity that may be altered by supplementation, but additional studies are necessary to determine whether these effects have clinical significance in terms of disease outcome. Vitamin A has been fairly well studied in terms of its immunomodulatory effects, and we will review the evidence from randomized controlled trials as well as potential mechanisms of action.

Vitamin A supplementation may afford some protection from infection in malnourished individuals, but the potential benefits of supplementation in normal wellÂnourished individuals remain to be established. There is evidence from several studies that suggests that vitamin A deficiency is associated with depressed immune function and an impaired response to influenza infection. Supplementation of vitamin A is associated with a reduction of mortality and morbidity among certain populations. It appears that populations suffering from malÂnutrition may benefit from adequate or additional vitamin A supplementation. However, it is less clear if normal, healthy, well-nourished individuals will benefit from additional supplementation with respect to enhanced immunity. The results from several studies involving beta-carotene supplementation in the diet of healthy individuals suggest that certain aspects of innate immunity, such as NK cytotoxicity and monocyte production of the cytokine TNFcx, are enhanced. It appears that lymphocyte subsets or the lymphocyte response to mitogens are not altered. In addition, one study of healthy older individuals found that vitamin A supplementation was associated with a reduction in the number of T lymphocytes. Whether these observed changes of immune function in response to supplementation actually result in reduced susceptibility to infection in healthy individuals is not well established. The results from one study demonstrated no association between vitamin A supplementation and incidence of bacterial infection. we are not aware of any long-term, randomized clinical trials that have evaluated the incidence of viral infection in response to supplementation with vitamin A alone. However, several studies have examined the possibility that supplementation with several multivitamins and or trace elements such as zinc, may alter susceptibility to infection. In general, the findings from these studies show no protection from infection in association with vitamin intake, but a slight decrease in the incidence of infection in those individuals consuming supplemental trace elements such as zinc and selenium. At this time, the potential benefits of vitamin A supplementation for healthy well-nourished individuals regarding susceptibility to infection remain to be established.

A high beta-carotene intake has also been associated with a reduced risk of cancer. Earlier epidemiological studies suggested a high natural (fruits and vegetables) intake of beta-carotene was associated with reduced risk of cancer. However, more recent studies have not observed any benefit of beta-carotene intake on incidence of cancer and two studies actually observed an increased incidence of lung cancer in those participants consuming beta-carotene supplements. The presence of other carotenoids in fruits and vegetables has been suggested to be the protective factor in regards to cancer incidence in the early epidemiological studies based on the findings from these recent studies, dietary supplementation with high doses of synthetic beta-carotene may be contraindicated for smokers. As a reminder, it has been known for some time that a high intake of vitamin A results in adverse effects (neurologic, dermatologic, musculoskeletal, gastrointestinal, birth defects) and the results from the most recent studies suggest a potential risk of high doses of synthetic beta-carotene in certain populations. At this time it is probably safest to follow the National Cancer Institute recommendations that suggest five or more servings of fruits and vegetables per day.

Immune Effects and Exercise

We are currently aware of only one study that has examined whether vitamin A supplementation is associated with a reduced incidence of infection in athletes. Several studies have shown that the risk of upper respiratory infection is increased following competition in marathons or ultramarathons However, vitamin A supplementation before marathon competition did not reduce the incidence of infection in the postrace period. Therefore, to our knowledge, vitamin A supplementation has not been associated with enhanced resistance to infection in healthy athletes.

Article Source : What Causes Allergies

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Both K Bakhru & Robert Baird are contributors for EditorialToday. The above articles have been edited for relevancy and timeliness. All write-ups, reviews, tips and guides published by EditorialToday.com and its partners or affiliates are for informational purposes only. They should not be used for any legal or any other type of advice. We do not endorse any author, contributor, writer or article posted by our team.

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