Hypersensitivity: Definition, Types, Causes, Symptoms, and Risk Factors

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- Updated by Sarah Qaiser
Medically reviewed by Dr Sidra Samad


The definition of hypersensitivity is when the immune system behaves abnormally towards a harmless antigen, causing a reaction in the body. There are four types of hypersensitivity, which are Type I, IgE antibody-mediated response; type II, IgG/IgM that causes a cytotoxic response; type III, immune complexes-mediated response; and type IV, cellular response-mediated delayed reaction. Examples of antigens include pollen, food, and drugs, which can cause the immune system to react. The symptoms of hypersensitivity include a rash, flaky skin, fever, facial swelling, and abnormalities in the white blood cell count. The treatment of hypersensitivity can consist of epinephrine (adrenaline) for anaphylaxis and anti-inflammatory drugs for autoimmune disorders. The risk factors for developing hypersensitivity include genetics, concomitant infection, such as HIV; concurrent illnesses, such as schizophrenia and opioid addiction; and drug reactions, which cause the immune system to be more sensitive.

What is Hypersensitivity?

Hypersensitivity is an exaggerated immune system response towards harmless antigens. Hypersensitivity can include reactions to food, animals, the environment, and drugs. There are four types of hypersensitivity. These include Type I, Type II, Type III, and Type IV hypersensitivity, according to Science Direct in 2022. Hypersensitivity reactions include asthma, anaphylaxis, autoimmune hemolytic anemia, rheumatoid arthritis, and contact dermatitis. Hypersensitivity reactions can often create tissue damage or cause the body internal problems.

How common is Hypersensitivity?

Hypersensitivity in its less severe forms is more common, affecting over 50 million people in the USA, according to the Asthma and Allergy Network in 2023. Food allergies affect males and females evenly, with food allergies higher in girls by only 0.1%. Respiratory allergies are more common in males than females by 2.9%. Skin allergies affect both genders almost equally, with women affected slightly more by 0.7%. It is estimated that roughly 10% of the world’s population is affected by drug allergies. The types of hypersensitivity reactions from these sources differ, including rashes, asthma, and anaphylaxis. 

What are the Causes of Hypersensitivity?

There are several causes of hypersensitivity, according to Merk Manuel in 2023. When understanding hypersensitivity vs allergy, it’s essential to note that allergies are a subset of hypersensitivity. However, not all hypersensitivity reactions are caused by allergens. The causes of hypersensitivity are listed below.

  • Seasonal allergens such as tree or grass pollen.
  • Drug allergens that cause an adverse reaction in the body
  • Food allergies that cause a reaction
  • Allergies that are present year-round, such as animal dander or mold
  • Touching allergens, such as latex
  • Reacting to insect bites or stings

What are the Symptoms of Hypersensitivity?

The symptoms of hypersensitivity are listed below.

  1. Red Rash - A red rash is when the skin becomes red, inflamed, and may be itchy.
  2. Flaky Skin - Flaky skin is when inflamed skin becomes too dry and begins to flake off.
  3. Fever - A fever is a rare condition where a reaction causes the body’s temperature to increase drastically.
  4. Facial Swelling - Facial swelling is where the immune system creates histamine, which causes the area around the eyes, mouth, and cheeks to swell.
  5. Tender Lymph Nodes - Tender lymph nodes are caused when the lymph nodes in the throat are filled with excessive fluid, leading to tenderness and soreness.
  6. Swollen Salivary Glands - Swollen saliva glands are when the glands in the mouth are filled with excess fluid, becoming inflamed.
  7. Dry Mouth - A dry mouth is where the saliva glands are inflamed and don’t produce as much fluid, resulting in a dry mouth.
  8. Abnormalities in White Blood Cell Count - Abnormalities in a white blood cell count result from chronic and intense inflammation from an allergen and cause the body to produce excess white blood cells in response.
  9. Headache - A headache is when there is swelling or inflammation in the head area, causing pain. It is commonly associated with seasonal allergies.
  10. Seizures - Seizures are where the body has involuntary movement due to uncontrolled electrical impulses from the brain and are likely connected with inflammation from allergies.
  11. Coma - A coma is when the body falls unconscious for a long time due to internal damage from prolonged hypersensitivity reactions.

Wondering which of the following is not considered a hypersensitivity reaction? Typically, Hashimoto thyroiditis, where T-Cells attack the thyroid glands in a Type IV Hypersensitivity reaction, starts out as hypersensitivity. However, this progresses to hypothyroidism, where the thyroid is too weak to produce enough hormones to keep the body balanced.

1. Red Rash With or Without Pus-Filled Bumps

A red rash is when the skin becomes swollen, itchy, and inflamed on the surface, either in a localized area or generalized across the body. A red rash is a symptom of hypersensitivity when the body reacts to an allergen, causing the blood vessels to dilate. These dilated blood vessels increase the blood flow to the affected area, causing a red rash. A red rash with bumps is sometimes known as hives. A red rash with pus-filled bumps is known as eczema. The causes of red rashes include chemicals, specific ingredients, cosmetics, and drugs, according to the MSD Manuel in 2022. To better understand an allergy vs hypersensitivity reaction, it’s important to remember that all allergic reactions are hypersensitivity reactions, but not all hypersensitivity reactions are caused by allergies. 

2. Flaky skin

Flaky skin is marked by patches of inflamed skin peeling at the outer layer, leaving the skin dry and flaky. Flaky skin is a symptom of hypersensitivity and helps us to understand what is hypersensitivity when the body reacts to an allergen. This reaction causes the skin to become red and inflamed. Skin inflamed for lengthy periods can’t retain moisture and becomes dry. This dryness can lead to flaky skin and the shedding of the topmost layer. Allergens to certain chemicals, such as a detergent, can cause strong reactions that can develop into flaky skin.

3. Fever

A fever is when the body’s internal temperature exceeds normal boundaries, and the person feels abnormally hot and weak. A fever is not commonly associated with hypersensitivity. However, when hypersensitivity of the immune system creates a reaction like a fever, it typically involves serum sickness. Serum sickness is a Type III hypersensitivity reaction, according to the NIH National Library of Medicine in 2023. Serum sickness includes fever, rash, and arthralgia, which is pain in a joint. A fever can sometimes be caused by reacting to drugs like penicillin or nonhuman proteins, such as the proteins within antisnake venom.

4. Facial swelling

Facial swelling is when the areas around the eyes, lips, and cheeks begin to swell, causing the skin to appear puffy and red. Facial swelling is a symptom of hypersensitivity when the body creates a protein called histamine to combat an antigen. Histamine causes the blood vessels to expand, which can result in facial swelling. Facial swelling can be an allergic reaction to certain foods, such as nuts or shellfish. Out of the hypersensitivity types, facial swelling falls under Type I. 

5. Tender lymph nodes

A person can feel tender lymph nodes when the lymph nodes around the throat and back of the head areas are filled with excessive fluid. This excess fluid causes the lymph nodes to swell, feeling sore and tender. Tender lymph nodes become a symptom of hypersensitivity when allergens such as pollen or irritants enter the body, causing the lymphatic system to activate in response. This activation causes the lymph nodes to fill with fluid that contains white blood cells designed to fight off the foreign pathogen (in this case, an allergen.) Of these hypersensitivity reaction types, tender lymph nodes are typically a part of the Type IV category of hypersensitivity or part of an inflammatory response.

6. Swollen saliva glands

Swollen saliva glands are when the glands inside the mouth become inflamed, which can inhibit their functioning. Swollen saliva glands can lead to a dry mouth and a bad taste in the mouth. This symptom of hypersensitivity happens when the body reacts to an allergen, causing the immune system to make the glands swell with excess fluid. Swollen saliva glands are rare but can sometimes be caused by a food allergen. Swollen salivary glands are a part of the type1 hypersensitivity reactions.

7. Dry mouth

A dry mouth is when there is a lack of saliva, moisture, or fluids in the mouth, which can lead to a strange taste in the mouth and dryness. A dry mouth becomes a symptom of hypersensitivity as a byproduct of swollen saliva glands. When the glands become swollen, they do not produce as much saliva, which can lead to dryness in the mouth. A dry mouth is a type 1 hypersensitivity reaction typically caused by seasonal allergies or food.

8. Abnormalities in your white blood cell counts

Abnormalities in the white blood cell count are when the body has either a too-high or two-low white blood cell count, which can indicate a hypersensitivity reaction. Abnormalities in the white blood cell count are a symptom of hypersensitivity due to the body overreacting to an allergen, causing intensive inflammation. High levels of inflammation mean more white blood cells are being called to the site where an allergen is, which can, in turn, cause more white blood cells to be produced. This production of extra white blood cells can increase the count and create an abnormality. This can help us better understand what causes hypersensitivity.

9. Headache

A headache is tenderness and pain in the head's front, sides, and back. A headache is a symptom of hypersensitivity due to the swelling of lymph nodes in the area and inflammation. A headache is typically a symptom of a type 1 allergic reaction, usually accompanied by a runny nose or cough. A cause of a headache can be seasonal allergens or airborne allergens like pet dander.

10. Seizures

Seizures are where the body has involuntary movements, such as spasms or limpness, due to uncontrolled brain activity between the brain and the body’s muscles. Seizures become hypersensitivity examples of symptoms due to a possible connection between inflammation and allergic diseases, according to the NIH National Library of Medicine in 2017. 

11. Coma

A coma is when the body can’t wake up and is in a perpetual state of unconsciousness. A coma is a symptom of hypersensitivity due to extreme damage to internal organs, which can cause internal bleeding to the body and brain. This damage can cause the body to shut down, resulting in a coma. All of the following are considered examples of type I hypersensitivity except a coma, such as asthma and anaphylaxis.

When Do Hypersensitivity Symptoms Usually Occur?

Hypersensitivity symptoms of type I, II, and IV are considered immediate response reactions when exposed to allergens. This hypersensitivity reaction definition can include asthma and anaphylaxis, typically occurring between 12 to 72 hours of exposure. This fast-acting response is mediated by antibodies of the immune system, according to the NIH National Library of Medicine in 2023.

What are the Risk Factors of Hypersensitivity?

Many risk factors of hypersensitivity help define the rheumatoid arthritis hypersensitivity type and other types. These risk factors are listed below.

  1. Genetics - Genetics can include hypersensitivity being passed down from parent to child and is a risk factor for developing hypersensitivity.
  2. Concomitant Infection - Concomitant infections include HIV and herpes, which are linked to being predisposed to hypersensitivity.
  3. Concurrent Illnesses - Concurrent illnesses cause the immune system to be weaker, making hypersensitivity reactions more likely.
  4. Drug Reaction - Drug reactions cause the immune system to be more sensitive, which can lead to hypersensitivity reactions later.

1. Genetics

Genetics have been linked to a likelihood of developing hypersensitivity if the previous generation has it. An offspring has a 33% chance of developing hypersensitivity if one of their parents also has hypersensitivity, according to research by Springer in 2009. These inherited traits can include reactions like asthma and anaphylaxis. An exaggerated life-threatening reaction to a previously encountered antigen is called anaphylaxis, a hypersensitivity reaction.

2. Concomitant infections

Concomitant infections have been linked to a hypersensitivity reaction. However, the research is unclear on the exact physiopathology that predisposes people with concomitant infections to hypersensitivity, according to the NIH National Library of Medicine in 2006. Concomitant infections include diseases like HIV and herpes.

3. Concurrent illnesses

Concurrent illnesses include a mental health disorder and an addiction. Concurrent illnesses cause the immune system to be weaker, which can trigger type I hypersensitivity reactions, among other types of reactions. Antigens that trigger allergic reactions are called type I hypersensitivity reactions.

4. Drug Reaction

Drug reactions are when the immune system is triggered by a drug in the body, reacting to its presence. Drug reactions are a risk factor for developing hypersensitivity due to an activation or sensitization of the immune system when the drug is present. This sensitization can lead to hypersensitivity allergy reactions later on.

What are the Types of Hypersensitivity Reactions?

The types of hypersensitivity reactions are listed below, along with examples of hypersensitivity reactions.

  1. Type I - Type I reactions typically involve shortness of breath and wheezing, such as asthma and anaphylaxis.
  2. Type II - Type II reactions typically involve tissue damage from antibodies, such as an immune system attack on lung and kidney-located antigens.
  3. Type III - Type III reactions involve immune complexes that cause arthritis in the tissue.
  4. Type IV - Type IV reactions are typically delayed and involve cell-mediated responses to allergens.

Type I: IgE antibody-mediated response.

Type ! is an IgE antibody-mediated response from the immune system in reaction to an antigen. Early symptoms of Type I include shortness of breath and wheezing. A few examples of these hypersensitivity reactions types include asthma, anaphylaxis, and allergic rhinitis. Diagnosing Type I is usually done with an allergy skin test administered by a doctor. Treating type I hypersensitivity symptoms, such as anaphylaxis, is usually done by administering epinephrine that counters the effects of blocked bronchial muscles.

Type II: Antibodies IgG/IgM that cause a cytotoxic response.

Type II is when IgG or IgM antibodies attack the antigens attached to cells. An example of Type II reactions include Goodpasture’s syndrome, where the immune system attacks the cells in the lungs and kidneys via antibodies, causing tissue damage. Drug provocation tests (DPT) are used to determine if a person has type II hypersensitivity. Type II hypersensitivity is treated with immunosuppressants that cause antibodies to stop behaving in unusual ways. To answer “Which of the following is not a possible symptom of type I hypersensitivity?”, it’s important to remember that the immune system attacking the body’s internal structures is not seen in type I reactions. These reactions are mainly only seen in type II reactions.

Type III: Immune complexes-mediated response

Type III reactions are an immune complexes-mediated response to antigens. Immune complexes consist of antigens and antibodies bound together, then are moved into the body's tissues, causing inflammation. An example of type III hypersensitivity includes rheumatoid arthritis.  If there are skin lesions, these can be biopsied to find the immune complexes. However, most diagnoses of type III hypersensitivity are determined through symptoms of fever, rash, and arthritis. Removing the allergen and providing antihistamines can help treat the type III reactions. When the body overreacts to a specific antigen, which is a hypersensitivity reaction, this is a(n) allergy reaction.

Type IV: Cellular response-mediated delayed reaction.

Type IV responses take longer to develop than the other reaction types and are cell-mediated responses. Examples of type IV reactions include allergic contact dermatitis and multiple sclerosis. To diagnose type IV hypersensitivity, skin patch tests may be performed by a doctor. Medications like corticosteroids are prescribed as treatments, and avoiding exposure to offending allergens. The answer to “What is the term for an exaggerated, abnormal reaction to an allergen?” would be a hypersensitivity reaction.

What are the Complications of Hypersensitivity?

The complications of hypersensitivity are listed below.

  • Status Asthmaticus - This acute bronchial asthma does not respond to standard treatments and requires immediate emergency action.
  • Anaphylactic Shock - This life-threatening reaction makes the bronchial muscles close around the airways.
  • Post-transfusion Reaction - This reaction happens around 24 hours after a blood transfusion. Symptoms include back pain, fever, and dyspnea.
  • Serum Sickness - This reaction happens after a drug has been administered. A hypersensitivity rash on the skin, fever, or arthritis are the main symptoms. It affects the body's organs, mediated by immune complexes.

How does Hypersensitivity affect the Body?

The different types of hypersensitivity affect the body in different ways. Type I affects the body through rashes, hives, and wheezing. Type II creates symptoms such as damage to cells and tissues and insufficient platelets in the body. Type III causes serum sickness and lupus in the body. Type IV affects the body through contact dermatitis. 

How does Hypersensitivity Affect the Brain?

Hypersensitivity affects the brain through brain fog due to the exhaustion the body is put through in reaction to allergens. Hypersensitivity also affects the brain through chronic fatigue, hormone imbalances, and depression. Hypersensitivity reactions examples include asthma, anaphylaxis, and arthritis. 

How does Hypersensitivity affect Lifestyle?

The type of hypersensitivity reactions may affect lifestyle by interfering with a person’s ability to complete daily tasks. They may also prevent a person from going outside and interacting with people socially. 

How is Hypersensitivity Being Diagnosed?

Hypersensitivity is diagnosed by collecting a patient's medical history, asking about the exposure to the allergen, and taking skin patch tests. The doctor may conduct biopsy tests on skin lesions if they are present. The doctor may also ask about symptoms and perform a complete physical examination. 

How is Hypersensitivity Prevented?

The best way to prevent hypersensitivity is to reduce or limit exposure to the allergen-causing problems. Type 2 hypersensitivity examples include Goodpasture’s syndrome, where antibodies attack antigens found on the cells located there, causing tissue damage. If no antigens are present, hypersensitivity can be prevented.

What are the Treatments available for Hypersensitivity?

Treating and managing hypersensitivity, such as anaphylaxis, includes the administration of epinephrine, antihistamines, and oxygen. Asthma can be treated with inhaled bronchodilators and inhaled corticosteroids and can help answer “Which of the following is an example of a type I hypersensitivity reaction?” Treating and managing hypersensitivity, such as autoimmune disorders, includes anti-inflammatory drugs. These treatments are reasonably effective for managing hypersensitivity. However, the best treatment is to prevent the introduction of hypersensitivity-causing allergens.

How Does Diphenhydramine Help with Hypersensitivity?

Diphenhydramine helps with hypersensitivity by reversing the effect of histamines on the body's capillaries. Diphenhydramine works as an antihistamine, effectively mitigating asthma and allergic hypersensitivity reactions. Antigens that induce an allergic reaction are called allergens.

What to Expect in Hypersensitivity Condition?

In a hypersensitivity condition, a person can expect to have allergic reactions or hypersensitivity reactions, which are abnormal immune responses to a harmless antigen. The types of hypersensitivity reactions and examples include sneezing and watery eyes for seasonal allergies. Some reactions are from food like nuts and cause anaphylaxis. 

How Long Does Hypersensitivity Last?

A hypersensitivity reaction can last up to 24 to 48 hours after treatment has stopped. Minor reactions, such as a rash, may still be experienced during desensitization. This can help answer “a reaction to an allergen is a ________ hypersensitivity.” This would be a type I reaction.  

Is hypersensitivity a mental illness?

No, hypersensitivity is not a mental illness. Studies have shown, however, that people with seasonal allergies are more susceptible to mental health disorders, according to the NIH National Library of Medicine in 2018. Type 2 vs type 3 hypersensitivity may impact a person’s mental health via fatigue and pain associated with both types of hypersensitivities.

Is Hypersensitivity a Serious Illness?

Yes, depending on the type of hypersensitivity involved. A type 1 hypersensitivity resulting in anaphylaxis may cause the airways to block, which is a medical emergency. A type 4 hypersensitivity involving Crohn's disease, a chronic Irritable Bowel Syndrome, can cause life-threatening complications, such as internal bleeding. These can help differentiate between a type 1 and a type 4 allergic reaction and how they affect a person.

What is the difference between Hypersensitivity and Autoimmunity?

The difference between hypersensitivity and autoimmunity is that hypersensitivity is a reaction to the outside, while autoimmunity is a reaction to the inside of the body. Another difference between hypersensitivity and autoimmunity is that hypersensitivity usually involves strong responses to harmless antigens. Autoimmunity, on the other hand, is when the immune system stops working correctly, which causes it to attack itself. All of the following are involved in type 2 hypersensitivity except for autoimmunity. 

What is the difference between Hypersensitivity and Allergy?

The difference between hypersensitivity and allergies is that all allergies are hypersensitivity reactions, but not all hypersensitivity reactions are allergies. Hypersensitivity reaction examples include reacting to drugs, which are not allergens. However, pollen is an allergen that can cause an allergy or a hypersensitivity reaction.