This type of hypersensitivity develops when the immune system encounters large number of free antigens.
Interaction of antibodies with free antigens results in the formation of precipitating antigen antibody complexes.
Normally monocytes or macrophages (MD) remove these precipitates very efficiently, but their efficiency is minimized, if the precipitate is formed in antigen excess resulting Type III hypersensitivity reactions.
The diseases brought about by Type III or immune complex mediated reactions is broadly classified into 3 categories
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1. Hypersensitive Reactions to Weak Abs
If the antigens are weak and fail to stimulate strong Ab response, Abs production in small quantities continues as long as the Ags are present inside the body.
The immune complexes formed by the continuously forming Abs and the weak Ags are deposited on surrounding tissue.
Generally parasites such as Plasmodium, Streptococcus etc. staying for a long period inside the host cause this kind of hypersensitivity.
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2. Production of Abs Against Auto Ags:
Some times immune system produces antibodies against certain auto antigens.
Since immune system is in continuous contact with their auto antigens, production of antibodies continues as long as the antigen processing cells are present in the body.
Continuous antibody production results in great loss of cells and lead to pathological changes.
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3. Excessive Ag-Ab Complex Formation:
When the scavenger systems such as phagocytes, complement system etc. get over loaded, certain Ag- Ab complexes are left in the body. The left out Ag- Ab complexes then settle on several surrounding tissues causing diseases like Systemic Lupus Erythematosus (SLE).