Allergy
This is a over powerful immune response to an antigen that is usually harmless, eg. house dust, animal fur, pollen. Thus the damage caused to the body is by the immune response rather than the antigen.
On initial exposure to the allergen, the individual becomes sensitised to it, and this leads to a massive response on later exposure. The reaction can be mild - running nose, itchy eyes, or it can be severe - anaphylactic shock.
The four types of hypersensitivity are below:
Hypersensitivity
Type One - Immediate, Anaphylactic
eg. food allergy, hay fever, eczema Individuals who have inherited high levels of immunoglobulin IgE can develop this. When exposed to an allergen, eg. house dust, the antibodies activate mast cells and basophils to degranulate. This leads to histamine being released, which causes smooth muscle constricts, vasodilation and increased vascular permeability - leading to tight airways, as in asthma and swelling, as in inflammation. This can be anaphylaxic, and can lead to death.
Type Two - Immediate, Cytotoxic
eg. autoimmune, haemolytic disease and transfusion reactions When an antibody binds with a cell that is foreign, it is marked for destruction (opsonisation), and hunted down by lymphocytes and other leukocytes. If the cell is not foreign, but self, an autoimmune response occurs, killing off the body's own cells. Also happens with wrongful blood transfusions, for example.
Type Three - 4-8 hours, immune complex mediated
eg. glomerulonephritis, penicillin allergy When immune complexes form (antigen-antibody), sometimes there are lots of them, and they float around the body getting deposited. For example, in the kidneys. This then triggers an immune response, with leukocytes attacking the kidney cells, getting confused cos they are covered with immune complex.
Type Four - 24-48 hours, Delayed-type
eg. tuberculosis, graft rejection, contact dermatitis, ie. nickel allergy Only one not to do with antibodies, but memory T cells. These overreact to an antigen, causing overload of cytoxic T cells.