Allergic reactions such as hay fever (seasonal allergic rhinitis) are caused by an oversensitivity or over-reaction of the immune system to a particular allergen. An allergen is a substance that is foreign to the body and which can cause an allergic reaction in certain people. For example, pollen, dander, mold, some germs. In most people, the immune reaction to these foreign substances is normal and appropriate. But in allergic people, it is excessive. For example, in people with hay fever, contact with pollen in the nose, throat and eyes triggers the mast cells there to release much more histamine than normal. This excessive release of histamine produces the associated symptoms of itching, swelling, runny eyes, etc.
Antihistamines work by physically blocking the H1 receptors, stopping histamine from reaching its target. This decreases your body's reaction to allergens and therefore helps to reduce the troublesome symptoms associated with allergy.
Histamine is a chemical naturally produced by various cells in your body. It has a variety of different functions. Large amounts of histamine are made in cells called mast cells, in places where the body comes into contact with the outside environment. For example, in the nose, throat, lungs and skin. Here, mast cells and histamine form part of your immune defence system.
Like most drugs used to treat infectious pathogens, antivirals are targeted to specific strains of viruses and work in a variety of ways. Most antiviral drugs don't actually kill the virus particles themselves as inhibit their reproduction. Since viruses cannot reproduce without infecting a host cell antiviral drugs have been designed to interfere with the infection process. This interference may be achieved in numerous ways, including blocking the virus from the host cell, preventing the virus from releasing its genetic material once it reaches the nucleus and preventing the virus's genetic data from being spliced into the host cell's DNA. Various highly specific antiviral drugs have also been developed that target the enzymes and proteins that an infected host cell uses to assemble new virus particles and prevent them from functioning correctly. Such drugs must be designed very carefully so that they do not interfere with the metabolism of healthy cells. A final type of antiviral drug targets the virus indirectly, by increasing the efficiency with which the host's immune system can fight the viral infection.