Hypersensitivity

Hypersensitivity
The body’s immune system is not a perfect system. Oftentimes, it will attack itself. There are many different scenarios and events that can cause a person’s immune system to attack its own cells or tissue. The term hypersensitivity is used to describe heightened states of immune responsiveness, and it breaks down into four main types (types 1-4).

Type 1:
Type 1 hypersensitivity is generally caused by cell bound antibody reacting with free antigen. Good examples are pollen and peanuts. Although very complex, in its simplest form, a small amount of antigen causes a large amount of IgE to be produced. That IgE will then bind to mast cells, and when re-exposed to the same antigen, the mast cells degranulate and release large amounts of histamine and other products. Conditions most commonly associated with type 1 hypersensitivity are: anaphylaxis, food allergies, pollen or plant allergies, and asthma.

An old test to be aware of, radioallergosorbent test (RAST) was the original test used to detect specific IgE types. RAST used a radioactive label which has been replaced by newer methods although the principles remain the same.

Type 2:
Type 2 hypersensitivity involves free antibody (IgG and IgM) binding to antigens on cell surfaces leading to cell death via opsonization or the complement system. The best example of this are transfusion reactions. Hemolytic disease of the newborn (HDN) and autoimmune hemolytic anemia will also fall into this category.
Type 3:
Type 3 hypersensitivity also involves antigen-antibody complexes. The antigens in type 3 are soluble, meaning when they bind to antibody they can precipitate out of the serum and sometimes end up in tissue. The complexes can activate complement and other processes and cause tissue damage. Examples of type 3 hypersensitivity are systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), and serum sickness.

Type 4:
Type 4 hypersensitivity is also known as delayed hypersensitivity because of the involvement of T-cells. In type 4, contact with an antigen will induce a sensitization phase (1-2 weeks) where T-cells are presented with the antigen. Upon re-exposure to the same antigen, T-cells activate and create an inflammatory response 4-6 hours after contact and peaking 2-3 days after contact. Example of type 4 hypersensitivity are tuberculosis, leprosy, graph vs host disease, and poison oak.

Nerdy Note
The rash from poison oak and poison ivy is caused by the oil urishiol. Urishiol is a small molecule that binds to skin cell protein and changes its shape. The shape change causes it to be engulfed by an antigen presenting cell called a Langerhans cell. This causes a T-cell activation which leads to an upregulation in cytokines and other products that cause the rash.

Study Tip
Using the phrase “All Dogs Can Dance” is an easy way to remember the different types of hypersensitivity. Using the first letter from each word (ADCD), can help you remember properties of the different types of hypersensitivity.

A = Anaphylactic
D = Cell Death
C = Complexes
D = Delayed