Question

Individuals with the autoimmune disease systemic lupus erythematosus (SLE) produce antibodies to DNA and phospholipids. (a) Explain why normal individuals do not make antibodies to these substances. (b) During a normal response to a viral or bacterial infection, the immune system produces large amounts of antigen-specific antibodies, and the resulting antigen–antibody complexes are subsequently removed from the circulation and degraded. Explain why antigen–antibody complexes accumulate in the tissues of individuals with SLE.

Short answer

(a)Normal individuals do not make antibodies to DNA and phospholipids. Antibodies are made against antigens, which are actually proteins. DNAs are nucleic acids, and phospholipids consist of fatty acids. (b) The antigen-antibody complexes accumulate in the tissues of individuals with SLE due to the inability to degrade such complexes.

Step-by-step solution

Autoimmune disease

The immune system makes antibodies in response to a foreign antigen molecule. But, in autoimmune diseases, the immune system cannot distinguish between the foreign and self-antigens and forms antibodies against the body’s own antigens.

(a) Normal individuals do not make antibodies to DNA and phospholipids

DNA resides in the cell nucleus, and phospholipids reside in the cell membrane. DNA carries the genetic materials needed for an organism to reproduce and develop. The phospholipids are essential in transporting and removing cholesterol, fat, etc.

The antibodies are generally made against foreign antigens, which are proteins. DNA or phospholipids are not proteins. Thus, the immune system does not make antibodies against such substances generally.

(b) Antigen-antibody complex accumulation in the tissues of individuals with SLE

In systemic lupus erythematosus, the immune system makes antibodies against the body’s own DNA and phospholipids. Large antigen-antibody complexes are formed when equimolar concentrations of antigen and antibody are present.

The inability to degrade such large complexes in the lysosomes leads to the deposition of those complexes at the immune cell surface. This deposition in a later stage leads to the accumulation of those antigen-antibody complexes in the tissues, specifically in the kidney.