This Month's Feature Review:

Recurrent Infections: Normal, Allergic, or Immunodeficiency?

Vincent St Aubyn Crump

The human immune system is a complex defence system, consisting of cells and proteins, which act together, to fight infections caused by bacteria, viruses, and parasites. We are constantly exposed to infectious agents, and everyone gets an infection once in a while. However, some people get recurrent infections. There are several reasons for these individuals to have an increased susceptibility to infections.

In assessing the child with recurrent infections one needs to remember the rarity of true immunodefeciency and the high prevalence of allergy (atopy), exposure to cigarette smoke, attendance at day care (with high level exposure to bugs), and anatomic variations as predisposing factors for infections...

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New and Interesting Allergy Studies:

An enteric helminth infection protects against an allergic response to dietary antigen.

Bashir ME, Andersen P, Fuss IJ, Shi HN, Nagler-Anderson C.

Mucosal Immunology Laboratory, Massachusetts General Hospital and Harvard Medical School, Charlestown, MA 02129, USA

Reference: J Immunol 2002 Sep 15;169(6):3284-92

This study looked at the effects of worm infestation in mice and its effect on them developing peanut anaphylaxis. The results suggest that worm infestation protects against allergy / anaphylaxis by blocking the production of allergen-specific IgE.

It seems as if the immune system needs some amount of infection in early life to assist in its normal programming for the future.

The 'Hygiene Hypothesis' proposes that overcrowding and unhygienic contacts early in life may protect from allergic (atopic) diseases by facilitating exposure to microbes. So the increase of allergic diseases in the industrialised world has been explained by a decline in infections during childhood. Several new studies have recently investigated the role of changes of human microbial flora, declining exposure to food-borne and orofecal (bacterial) infections, to helminth (parasites) and to environmental sources of endotoxin as putative contributors to the rise of allergy and asthma cases in developed countries.

The immunological explanation for the hygiene theory is that the "clean" change in the environment has disrupted the workings of the immune system. Among the white blood cells that protect the body, there are two kinds of lymphocytes that interact in a kind of feedback mechanism. There is the T-Helper 1 (TH1), the kind that fights intracellular infections (like viruses) and the T-Helper 2 (TH2), the kind that fights extracellular infections (like parasitic worms) and, erroneously, allergens. In a healthy body, as the production of one kind of cell is triggered, a protein is released that suppresses the production of the other kind, and vice versa. With allergies the immune system has lost its capacity to keep both arms of the defence system in check.

It has been argued that bacterial and viral infections during early life direct the maturing immune system toward TH1, which counterbalance pro-allergic responses of TH2 cells. Thus, a reduction in the overall microbial burden will result in weak TH1 imprinting and unrestrained TH2 responses that allow an increase in allergy.