hypersensitivity reactions are excessive immune responses to normally harmless substances that induces inflammation and leads to tissue damage and death there are four types of hypersensitive reactions type 3 reactions are caused by an antigen antibody reaction which we call an immune complex this is different to type 1 reactions where we saw fixed antibodies on mast cells and freely floating antigens where in type 2 reactions we saw a fixed antigen on the cell and freely float in antibodies so in type 3 reactions because the immune complex forms within the circulation the harmful effects is not where they form rather it's where they deposit an example here in the tissue bed when the immune complex forms it brings in complement proteins which binds to the complex and then brings in neutrophils or macrophages which then release their lysozymes causing inflammation and destruction to tissue so once the immune complex has formed in the circulation the common locations at tissues where they will deposits are at blood vessels which will lead to vasculitis in kidneys which leads to nephritis at joints which causes arthritis or in skin causing dermatitis or in the lungs where we call pneumonitis so some common examples would be post a streptococcal infection this could lead to a complex lead into post strep glomerulonephritis rheumatoid or reactive arthritis or two systemic examples lupus being one which can deposit in the skin in the kidney and in the joint causing arthritis nephritis and dermatitis or serum sickness which can get stuck in the joints causing arthritis in the blood vessels causing vasculitis or in the kidneys causing nephritis