In psoriatic arthritis, arthritis means joint inflammation, and psoriatic refers to psoriasis, which is an autoimmune disease characterized by red scaly patches in the skin.
So psoriatic arthritis is a type of joint inflammation that happens in individuals with psoriasis.
Psoriatic arthritis is also one disease in a group of diseases called seronegative spondyloarthropathies.
Spondyloarthropathies are autoimmune diseases that affect the joints, and they’re seronegative, meaning that there aren’t any specific autoantibodies linked to them.
Normally, immune cells are ready to spot and destroy anything foreign that could cause the body harm.
To help with this, most cells express the gene HLA-B27, which encodes a protein that forms a major histocompatibility complex, or MHC, class I molecule that sits on the surface of the cell membrane.
This MHC class I molecule acts like a serving platter, presenting molecules from within the cell for the immune system to sample.
A CD8+ T-cell, also called a cytotoxic T-cell, uses its T-cell receptor to bind to the antigen presented by the MHC class I molecule.
Normally, the antigen that’s presented is from the cell, and the immune system recognizes it as a harmless self-antigen, which leads to no response.
Now, many individuals with psoriatic arthritis have a specific version of the gene HLA-B27, which somehow leads to an autoimmune process.
In these individuals, the immune system attacks self-antigens specifically ones in the joints.
Exactly what causes this is unclear, but it's clear that the gene is not enough to trigger psoriatic arthritis.