Having a hypersensitivity means that someone’s immune system has reacted to something in a way that ends up damaging them, as opposed to protecting them.
There are four different hypersensitivities and the third type or type III hypersensitivity reaction happens when antigen-antibody complexes deposit in blood vessel walls, causing inflammation and tissue damage.
Alright so first off, type III hypersensitivity reactions are mediated by immune complexes. Immune complexes, aka antigen-antibody complexes are made of two parts—the antigen and the antibody.
Antibodies, sometimes called immunoglobulins, are produced by plasma cells, which are basically fully matured and differentiated B cells.
Initially these cells make IgM - which can be secreted or bound to the plasma cell surface where it acts as a B cell receptor.
When a B cell undergoes cross-linking of two surface bound IgMs, it then takes up the antigen and presents a piece of it to T helper cells via t cell receptor to the MHC- class II molecule presenting the piece of antigen, along with costimulatory molecule CD4.
The B cell’s CD40 also binds to the T cell’s CD40 ligand, and then the t cell releases cytokines, which results in b cell activation and class switching, or isotype switching, where it changes the type of antibodies it makes.
In type III hypersensitivity reactions, typically B cells will switch from making IgM to making IgG antibodies.