Reactive arthritis

Reactive arthritis, also known as Reiter’s syndrome, is inflammation of a joint which usually develops after an infection, and that infection is typically a sexually transmitted disease or gastroenteritis.

Reactive arthritis is part of a group of diseases called seronegative spondyloarthropathies.

Spondyloarthropathies are autoimmune, inflammatory joint diseases, and they’re seronegative, which refers to the fact that an auto-antibody called rheumatoid factor is absent from the blood.

Normally, the immune cells are ready to spot and destroy anything foreign that could cause the body harm.

To help with this, most cells in the body have a set of proteins that combine together to form something called a major histocompatibility complex, or MHC, a molecule that sits on the surface of their cell membrane.

There are two kinds, class I and class II.

Class I molecules are found on most cells in the body, and they present molecules from within the cell for the immune system to continually sample.

Normally the molecule’s just a sample from the cell, a self-antigen. When immune cells pass by, they recognize it as harmless so there’s no response.

Class II molecules are found specifically on phagocytic cells like macrophages which destroy and digest foreign pathogens like bacteria.

Once a macrophage destroys a bacterium, it presents a piece of that bacterium on it’s MHC class II receptor, and the macrophage then makes its way to the lymph node to find some T-lymphocytes.

A type of T-lymphocyte, called a CD4+ T-cell, also known as a helper T-cell, uses its T-cell receptors to bind to the foreign antigen presented by the MHC class II molecule.

If the helper T-cell binds strongly, the antigen is recognized as foreign, and the helper T-cell switches on the corresponding B-cell, so it can start producing a whole lot of antibodies.

These antibodies bind to the specific pathogen, and typically prevent it from attacking the host’s cells and, at the same time, “tag” the pathogen for further destruction by other immune cells.

Reactive arthritis can develop after a sexually transmitted infection like chlamydia, or after gastroenteritis caused by bacteria like salmonella, shigella, yersinia, campylobacter, and E.coli.

These are bacteria are all gram negative, so they have molecules on their outer surface called lipopolysaccharides.

These molecules produce a strong immune response, so much so, that their other name is endotoxin.

Also, most individuals with reactive arthritis have the specific gene HLA-B27 which is one of many genes that contains code to make MHC class I receptors.

HLA-B27 is also associated with other inflammatory diseases like psoriasis and ankylosing spondylitis.

Exactly how reactive arthritis develops is unclear, but it probably has to do with the way that highly reactive lipopolysaccharides react with the MHC molecules on a cell surface.