Hippuric acid has long been used as an indicator of toluene exposure; however, there appears to be some doubt about its validity. There is significant endogenous hippuric acid production by humans; which shows inter- and intra-individual variation influenced by factors such as diet, medical treatment, alcohol consumption, etc. This suggests that hippuric acid may be an unreliable indicator of toluene exposure. It has been suggested that urinary hippuric acid, the traditional marker of toluene exposure is simply not sensitive enough to separate the exposed from the non-exposed. This has led to the investigation of other metabolites as markers for toluene exposure.

Urinary "o"-cresol may be more reliable for the biomonitoring of toluene exposure because, unlike hippuric acid, "o"-cresol is not found at detectable levels in unexposed subjects. o-Cresol may be a less sensitive marker of toluene exposure than hippuric acid. o-Cresol excretion may be an unreliable method for measuring toluene exposure because o-cresol makes up <1% of total toluene elimination.

Benzylmercapturic acid, a minor metabolite of toluene, is produced from benzaldehyde. In more recent years, studies have suggested the use of urinary benzylmercapturic acid as the best marker for toluene exposure, because: it is not detected in non-exposed subjects; it is more sensitive than hippuric acid at low concentrations; it is not affected by eating or drinking; it can detect toluene exposure down to approximately 15 ppm; and it shows a better quantitative relationship with toluene than hippuric acid or "o"-cresol.

Serious adverse behavioural effects are often associated with chronic occupational exposure and toluene abuse related to the deliberate inhalation of solvents. Long-term toluene exposure is often associated with effects such as: psychoorganic syndrome; visual evoked potential (VEP) abnormality; toxic polyneuropathy, cerebellar, cognitive, and pyramidal dysfunctions; optic atrophy; and brain lesions.

The neurotoxic effects of long-term use (in particular repeated withdrawals) of toluene may cause postural tremors by upregulating GABA receptors within the cerebellar cortex. Treatment with GABA agonists such as benzodiazepines provide some relief from toluene-induced tremor and ataxia. An alternative to drug treatment is vim thalamotomy. The tremors associated with toluene misuse do not seem to be a transient symptom, but an irreversible and progressive symptom which continues after solvent abuse has been discontinued.

There is some evidence that low-level toluene exposure may cause disruption in the differentiation of astrocyte precursor cells. This does not appear to be a major hazard to adults; however, exposure of pregnant women to toluene during critical stages of fetal development could cause serious disruption to neuronal development.

This remains a challenge in clinical practice due to a lack of reliable markers. Many other conditions lead to similar clinical as well as pathological pictures. To diagnose hepatotoxicity, a causal relationship between the use of the toxin or drug and subsequent liver damage has to be established, but might be difficult, especially when idiosyncratic reaction is suspected. Simultaneous use of multiple drugs may add to the complexity. As in acetaminophen toxicity, well established, dose-dependent, pharmacological hepatotoxicity is easier to spot. Several clinical scales such as CIOMS/RUCAM scale and Maria and Victorino criteria have been proposed to establish causal relationship between offending drug and liver damage. CIOMS/RUCAM scale involves a scoring system that categorizes the suspicion into "definite or highly probable" (score > 8), “probable” (score 6-8), “possible” (score 3-5), “unlikely” (score 1-2) and “excluded” (score ≤ 0). In clinical practice, physicians put more emphasis on the presence or absence of similarity between the biochemical profile of the patient and known biochemical profile of the suspected toxicity (e.g., cholestatic damage in amoxycillin-clauvonic acid ).

In most cases, liver function will return to normal if the offending drug is stopped early. Additionally, the patient may require supportive treatment. In acetaminophen toxicity, however, the initial insult can be fatal. Fulminant hepatic failure from drug-induced hepatotoxicity may require liver transplantation. In the past, glucocorticoids in allergic features and ursodeoxycholic acid in cholestatic cases had been used, but there is no good evidence to support their effectiveness.

An elevation in serum bilirubin level of more than 2 times ULN with associated transaminase rise is an ominous sign. This indicates severe hepatotoxicity and is likely to lead to mortality in 10% to 15% of patients, especially if the offending drug is not stopped (Hy's Law). This is because it requires significant damage to the liver to impair bilirubin excretion, hence minor impairment (in the absence of biliary obstruction or Gilbert syndrome) would not lead to jaundice. Other poor predictors of outcome are old age, female sex, high AST.

A literature review of 2014 found that non-coeliac gluten sensitivity diagnosis can be reached only by excluding celiac disease (CD) and wheat allergy.

Persons suspected of having celiac disease may undergo serological testing for IgA anti-tissue transglutaminase antibodies (abbreviated anti-tTG antibodies or anti-TG2 antibodies) and anti-endomysial antibodies (abbreviated EMA) provided the IgA-level is high, and if IgA is low, testing for certain IgG antibodies; in case of positive serological indication, a duodenal biopsy may confirm active celiac disease.

Eliminating the possibility of CD can generally also be done by adding HLA-DQ typing. The absence of HLA-DQ2 and HLA-DQ8 has a very high negative predictive value for CD, and the predictive value can be further enhanced by including HLA-DQ7.5 (HLA-DQ2 and HLA-DQ8 are found in coeliac disease 98% of the time in Caucasians, HLA-DQ7.5 present in the remaining 1.6% and only 0.4% of Caucasians are missed with the combination of these 3). Without serological or HLA-DQ2/8 positivity, celiac disease is likely not present. HLA-DQ typing has a practical advantage in that it is the only diagnostic test that allows to exclude CD when a patient is already on a gluten-free diet; however, as not only celiacs are HLA-DQ2/HLA-DQ8 positive, this method has a higher false positive rate than anti-TG2 and EMA antibody testing.

A four-of-five rule was proposed 2010 for confirming celiac disease, with the disease confirmed if at least four of the following five criteria are satisfied:

- typical symptoms of celiac disease;

- positivity of serum celiac disease immunoglobulin, A class autoantibodies at high titer;

- human leukocyte antigen (HLA)-DQ2 or DQ8 genotypes;

- celiac enteropathy at the small bowel biopsy; and

- response to the gluten-free diet.

For diagnosis of wheat allergy, allergy tests are available.

For patients with celiac disease, a lifelong strict gluten-free diet is the only effective treatment to date; for patients diagnosed with NCGS, there are still open questions concerning for example the duration of such a diet; for patients with wheat allergy, the individual average is six years of gluten-free diet, excepting persons with anaphylaxis, for whom the diet is to be wheat-free for life.

A gluten-free diet should not be started before the tests for excluding celiac disease have been performed, for the reason that the serological and biopsy tests for celiac disease are reliable only if the patient is consuming gluten.

Preferably, newly diagnosed celiacs seek the help of a dietician to receive support for identifying hidden sources of gluten, planning balanced meals, reading labels, food shopping, dining out, and dining during travel. Knowledge of hidden sources of gluten is important for celiac disease patients as they need to be very strict regarding eating only gluten-free food; for NCGS patients, it is not certain how strict the diet needs to be. Balanced eating is important because unless particular care is taken, a gluten-free diet can be lacking in vitamins, minerals, and fiber, and be too high in fat and calories.

The inclusion of oats in gluten-free diets remains controversial. Avenin present in oats may also be toxic for coeliac sufferers. Its toxicity depends on the cultivar consumed. Furthermore, oats are frequently cross-contaminated with gluten-containing cereals.

Lichen planus has a unique microscopic appearance that is similar between cutaneous, mucosal and oral. A Periodic acid-Schiff stain of the biopsy may be used to visualise the specimen. Histological features seen include:

- thickening of the stratum corneum both with nuclei present (parakeratosis) and without (orthokeratosis). Parakeratosis is more common in oral variants of lichen planus.

- thickening of the stratum granulosum

- thickening of the stratum spinosum (acanthosis) with formation of colloid bodies (also known as Civatte bodies, Sabouraud bodies) that may stretch down to the lamina propria.

- liquefactive degeneration of the stratum basale, with separation from the underlying lamina propria, as a result of desmosome loss, creating small spaces (Max Joseph spaces).

- Infiltration of T cells in a band-like pattern into the dermis "hugging" the basal layer.

- Development of a "saw-tooth" appearance of the rete pegs, which is much more common in non-oral forms of lichen planus.

The differential diagnosis for OLP includes:

- Other oral vesiculo-ulcerative conditions such as Pemphigus vulgaris and Benign mucous membrane pemphigoid

- Lupus erythematosus, with lesions more commonly occur on the palate and appear as centrally ulcerated or erythematous with radiating white striae. In contrast, OLP and lichenoid reactions rarely occur on the palate, and the striae are randomly arranged rather than radial.

- Chronic ulcerative stomatitis

- Frictional keratosis and Morsicatio buccarum (chronic cheek biting)

- Oral leukoplakia

- Oral candidiasis

The T helper cells (T cells) are a type of T cell that play an important role in the immune system, particularly in the adaptive immune system. They help the activity of other immune cells by releasing T cell cytokines. These cells help suppress or regulate immune responses. They are essential in B cell antibody class switching, in the activation and growth of cytotoxic T cells, and in maximizing bactericidal activity of phagocytes such as macrophages.

Mature T cells express the surface protein CD4 and are referred to as CD4 T cells. Such CD4 T cells are generally treated as having a pre-defined role as helper T cells within the immune system. For example, when an antigen-presenting cell expresses an antigen on MHC class II, a CD4 cell will aid those cells through a combination of cell to cell interactions (e.g. CD40 (protein) and CD40L) and through cytokines.

CD154, also called CD40 ligand or CD40L, is a cell surface protein that mediates T cell helper function in a contact-dependent process and is a member of the TNF superfamily of molecules. It binds to CD40 on antigen-presenting cells (APC), which leads to many effects depending on the target cell type. CD154 acts as a costimulatory molecule and is particularly important on a subset of T cells called T follicular helper cells (T cells). On T cells, CD154 promotes B cell maturation and function by engaging CD40 on the B cell surface and therefore facilitating cell-cell communication. A defect in this gene results in an inability to undergo immunoglobulin class switching and is associated with hyper IgM syndrome. Absence of CD154 also stops the formation of germinal centers and therefore prohibiting antibody affinity maturation, an important process in the adaptive immune system.

The importance of helper T cells can be seen from HIV, a virus that primarily infects CD4 T cells. In the advanced stages of HIV infection, loss of functional CD4 T cells leads to the symptomatic stage of infection known as the acquired immunodeficiency syndrome (AIDS). When the HIV virus is detected early in blood or other bodily fluids, continuous therapy can delay the time at which this fall happens. Therapy can also better manage the course of AIDS if and when it occurs. There are other rare disorders such as lymphocytopenia which result in the absence or dysfunction of CD4 T cells. These disorders produce similar symptoms, many of which are fatal.

The immune system must achieve a balance of sensitivity in order to respond to foreign antigens without responding to the antigens of the host itself. When the immune system responds to very low levels of antigen that it usually shouldn't respond to, a hypersensitivity response occurs. Hypersensitivity is believed to be the cause of allergy and some auto-immune disease.

Hypersensitivity reactions can be divided into four types:

- Type 1 hypersensitivity includes common immune disorders such as asthma, allergic rhinitis (hay fever), eczema, urticaria (hives) and anaphylaxis. These reactions all involve IgE antibodies, which require a T2 response during helper T cell development. Preventive treatments, such as corticosteroids and montelukast, focus on suppressing mast cells or other allergic cells; T cells do not play a primary role during the actual inflammatory response. It's important to note that the numeral allocation of hypersensitivity "types" does not correlate (and is completely unrelated) to the "response" in the T model.

- Type 2 and Type 3 hypersensitivity both involve complications from auto-immune or low affinity antibodies. In both of these reactions, T cells may play an accomplice role in generating these auto-specific antibodies, although some of these reactions under Type 2 hypersensitivity would be considered normal in a healthy immune system (for example, Rhesus factor reactions during child-birth is a normal immune response against child antigens). The understanding of the role of helper T cells in these responses is limited but it is generally thought that T2 cytokines would promote such disorders. For example, studies have suggested that lupus (SLE) and other auto-immune diseases of similar nature can be linked to the production of T2 cytokines.

- Type 4 hypersensitivity, also known as delayed type hypersensitivity, are caused via the over-stimulation of immune cells, commonly lymphocytes and macrophages, resulting in chronic inflammation and cytokine release. Antibodies do not play a direct role in this allergy type. T cells play an important role in this hypersensitivity, as they activate against the stimulus itself and promote the activation of other cells; particularly macrophages via T1 cytokines.

Other cellular hypersensitivities include cytotoxic T cell mediated auto-immune disease, and a similar phenomenon; transplant rejection. Helper T cells are required to fuel the development of these diseases. In order to create sufficient auto-reactive killer T cells, interleukin-2 must be produced, and this is supplied by CD4 T cells. CD4 T cells can also stimulate cells such as natural killer cells and macrophages via cytokines such as interferon-gamma, encouraging these cytotoxic cells to kill host cells in certain circumstances.

The mechanism that killer T cells use during auto-immunity is almost identical to their response against viruses, and some viruses have been accused of causing auto-immune diseases such as Type 1 diabetes mellitus. Cellular auto-immune disease occurs because the host antigen recognition systems fail, and the immune system believes, by mistake, that a host antigen is foreign. As a result, the CD8 T cells treat the host cell presenting that antigen as infected, and go on to destroy all host cells (or in the case of transplant rejection, transplant organ) that express that antigen.

Some of this section is a simplification. Many auto-immune diseases are more complex. A well-known example is rheumatoid arthritis, where both antibodies and immune cells are known to play a role in the pathology. Generally the immunology of most auto-immune diseases is not well understood.