FSGS and IgM nephropathy lead to nephrotic syndrome – the kidneys leak a lot of protein into urine. This leads to a drop in the levels of protein in the blood, which causes swelling in the body (oedema). 

Read more about why nephrotic syndrome happens

How the kidney works

Inside each kidney, there are about one million nephrons. Each nephron is made up of a glomerulus (when we talk about more than one glomerulus, we say glomeruli), and a renal tubule

  • Each glomerulus acts like a sieve, helping to remove extra water and waste from the body, and holding on to blood cells and protein, which the body needs.
  • Blood flows into the kidneys and to each glomerulus.
  • Most of the water and some other substances in the blood pass through the glomeruli.
  • This liquid flows into the renal tubule. Most of this liquid moves back into the bloodstream. The rest of it becomes urine.
  • The urine leaves the kidney by the ureters and goes into the bladder, where it is stored until we are ready to go to the toilet.

About the urinary system and kidneys – what the kidney does

Protein leak

Protein is an important part of our diet and is in most foods. When we eat protein, it is digested (broken down) in the stomach and intestines and taken into the blood. It is important that we have the right amounts of protein in our body to stay healthy.

Normally, most protein stays in the blood and does not get removed by the kidneys into the urine. This is mostly because proteins are too big to pass through the glomeruli.

In nephrotic syndrome, there is a change in the glomeruli. They become more leaky, and so some proteins pass through, leaving the body in urine. The main protein that leaks out is albumin which is quite a small protein and more likely to leak through the glomeruli.

Who gets FSGS and IgM nephropathy?

FSGS and IgM nephropathy is rare. About 1 in 50,000 children get nephrotic syndrome each year, but only about 1 in 10 of these cases are due to these conditions.

Some children who have had frequently relapsing nephrotic syndrome can develop FSGS or IgM nephropathy after many years. 

It is not always known why some children get FSGS or IgM nephropathy. It is often considered to be linked to a problem with the immune system, which is the body’s way of protecting itself from infection.

The immune system

Studies show that different types of nephrotic syndrome are linked to the immune system, which normally protects the body against disease and infection. Many germs – including bacteria and viruses – can make us sick if they get into the body. The immune system can kill these germs. However, if the immune system is not working properly, it can start to cause problems. In nephrotic syndrome, the immune system can interfere with the tiny filters in the kidneys, the glomeruli.

Read more about the immune system


The immune system protects the body against germs such as bacteria and viruses that can cause illness. These germs can enter the body in lots of ways, such as by the nose and throat or the urinary system. If we get a cold or flu, this means that a virus germ has got into the body and started to infect some of our body’s cells.

Germs have special ‘markers’ that are different from the markers on our own body’s cells. This means that the immune system can recognise that they are germs and kill them. We often feel sick for a few days or a few weeks while this is happening.

Parts of the immune system

The immune system has many different ways to protect the body against disease. Some of the parts include those listed below.

  • White blood cells are living cells in the blood. Often, the number of white blood cells found in a blood test can give information about someone’s immune system. The two main types of white blood cells in the immune system are neutrophils and lymphocytes.
  • Antibodies or immunoglobulins recognise the germs that have come into the body, and can bind (stick) to them. There are five types: immunoglobulin A (IgA), immunoglobulin D (IgD), immunoglobulin E (IgE), immunoglobulin G (IgG) and immunoglobulin M (IgM).
  • The complement system is made up of other proteins that float in the blood. These proteins work with (complement) other parts of the immune system to help kill germs or cells infected by germs. Normally, the body controls when complement is activated, so it does not attack the body itself.

When the immune system does not work properly

Sometimes the immune system does not work as expected and can cause problems. For example, sometimes the immune system cannot recognise the body’s own cells and may attack them as if they were invaders like germs.

Other causes

Some cases of FSGS and IgM nephropathy are due to other reasons, though these are more rare.

  • Genetic mutation – some children have a genetic mutation that affects the structure of part of the glomerulus in the kidney, which causes the protein leak. Genetic mutations are problems in the genes (which are in each of our body’s living cells), which are passed on by the parents. Your doctor will talk to you about this, and may arrange for a genetic test to be done.

Will it affect other family members?

It is very rare for FSGS, and even rarer for IgM nephropathy, to run in families. Your child’s doctor will ask about any family history of these conditions, and it is usual to arrange for siblings to have a urine test to check for protein.