Understanding Hypersensitivity, HIV Infection, and Transplant Rejection
Hypersensitivity
Hypersensitivity is an exaggerated immune response that, instead of protecting the body, causes multiple alterations.
Immediate Hypersensitivity Type I: Allergy
Known as allergy, this is a very quick response that appears 15 to 20 minutes after contact with the antigen (allergen). Pollen allergens can be an example. The allergic reaction takes place in three phases:
- Awareness Phase: When the body first comes into contact with the allergen, macrophages may engulf it and display its fragments on their surface via MHC proteins. The recognition of T helper cells, which are anchored to them, releases lymphokines that cause the maturation of neighboring B cells. These are transformed into plasma cells and release large amounts of IgE. The IgE binds to and coats the surface of mast cells in tissues (connective tissue mast cells) and blood basophils. This phase occurs without symptoms.
- Mast Cell Activation Phase: This is a release of chemical mediators such as histamine, serotonin, or prostaglandin.
- Allergy Phase: The release of chemical mediators causes allergy symptoms, such as swollen eyelids. Treatment involves using antihistamines, and the patient may undergo a desensitization process.
HIV Infection
HIV infection develops in two phases: the asymptomatic phase and the AIDS phase.
Reproductive Cycle
When the virus comes into contact with T helper cells, the envelope glycoprotein of the virus binds to the membranes of these cells. There is a membrane fusion by which the virus capsid is released into the cytoplasm of the cell. Proteins are reabsorbed, releasing their RNA, which, thanks to reverse transcriptase, copies DNA. The strands of RNA and DNA disappear, and the DNA copies itself and moves to the cell’s nucleus, integrating into its genome. During the asymptomatic phase, each time the cell divides, it transmits a copy of the viral DNA to each of the daughter cells. Antibodies against the virus can be detected in serum, and one speaks of a seropositive individual. The viral DNA is separated from the cell genome and expresses forms of RNA and mRNA molecules that migrate to the cytoplasm, where proteins are encoded in the capsid. After assembly with their respective pieces of RNA, the virus tries to leave the cell. These viruses infect other cells. The number of T lymphocytes decreases until it is unable to generate cellular immune responses. Then begins the AIDS symptomatic stage or phase in which the immune system is so weakened that microbial infections are widespread. The time between infection and the appearance of the first symptoms can range from one to ten years.
Transplant Rejection
The rejection mechanism is due to the activation of the recipient’s immune system. When tissue or an organ from a donor endowed with a specific surface antigen in their cells is implanted into a recipient, their T lymphocytes do not recognize it as their own and trigger an immune response against it. This causes the invasion of the transplant by a large number of cytotoxic lymphocytes and macrophages, which are cells that cause rejection. Several processes are then produced, which result in necrosis of the transplanted organ or tissue. Cytotoxic lymphocytes activate macrophages, and NK cells also secrete perforin that attacks cell membranes and destroys the transplant. Neutrophils also phagocytose opsonized cells, and platelets form clots. Antibody production in response to MHC antigens activates the complement system, causing cell lysis. Rejection is based on the differences in surface antigens of the donor and recipient. Autotransplants and isotransplants are considered safe. To avoid rejection, immunosuppressive drugs are used to lower the immune system response.