When the immune system overreacts
An allergy is an overreaction of the immune system to a normally harmless substance called an allergen (such as pollen, dust or certain foods). During an allergic reaction, cells release histamine, causing sneezing, itching, a runny nose or a rash. Symptoms may be mild, such as sneezing, or severe. A severe, whole-body reaction is called anaphylaxis and can be life-threatening. Common allergies include hay fever, asthma triggered by dust, and reactions to foods such as peanuts or seafood.
Remember
An allergen is harmless to most people; the problem is the immune system's overreaction, not the substance itself.
When the immune system attacks the body
In an autoimmune disease, the immune system mistakes the body's own cells for foreign antigens and attacks them. Examples include rheumatoid arthritis (attacking the joints), type 1 diabetes (attacking insulin-producing cells) and lupus. These conditions are usually long-term and are managed with medicines rather than fully cured.
When the immune system is too weak
Immunodeficiency means the immune system is weakened and cannot fight infection well. The virus HIV attacks helper T lymphocytes (a type of white blood cell); as these are destroyed, the immune system fails, leading to AIDS, in which even minor infections become dangerous.
Transplant rejection
After an organ transplant, the immune system may recognise the donor tissue as foreign (its antigens differ) and attack it. To reduce this rejection, patients are given immunosuppressant drugs and donors are matched as closely as possible; a close tissue match, often from a relative, lowers the chance of rejection. HIV, meanwhile, spreads through blood and body fluids, so infection can be reduced by safe practices.
Example
A person allergic to peanuts may suffer anaphylaxis within minutes of exposure, needing an adrenaline injection to reverse the swelling and open the airways.