HSP is a systemic condition – this means it affects different parts of the body. It is a type of vasculitis – the blood vessels are inflamed (swollen) and leak fluid into the tissues around the blood vessels.
HSP gets its name from two German doctors who described the condition – Edouard Henoch and Johan Schönlein.
After infection
Many children with HSP first get symptoms with an infection, caused by bacteria or viruses (germs). This infection is not in the kidney, but usually in the throat or tonsils. Most children recover from these infections. But, in a few children the infection seems to trigger HSP.
The immune system
HSP seems to be linked to the immune system, which normally protects the body from infections and disease.
A protein called immunoglobulin A (IgA), which is made by the immune system, gets trapped in small blood vessels, causing the swelling.
Germs
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.
How the kidneys are involved in HSP
In about half of children with HSP, the kidneys are affected. This is sometimes called HSP nephritis (‘nephritis’ means kidney disease).
The kidneys have many filters that clean blood to remove water, salt and waste into urine. These are made of up of tiny blood vessels called glomeruli.
In HSP, immunoglobulin A (IgA) is deposited (placed) on the filters, causing them to swell. They leak proteins and blood cells into urine.
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.
What happens in HSP
A protein called immunoglobulin A (IgA) gets trapped in the glomeruli. This causes the glomeruli to become inflamed (swollen) or damaged. The damaged glomeruli can let blood cells and protein through, which means they go into the urine.
IgA nephropathy
Doctors think that HSP with kidney involvement is related to the kidney condition, IgA nephropathy – the same proteins (immunoglobulin A) are trapped in the glomeruli. However, in IgA nephropathy only the kidneys are involved, and children do not have a skin rash, joint pain or abdominal pain.
Will HSP affect other family members?
HSP does not usually run in families. If one of your children has this condition, it is unlikely that another of your children or another family member will get it.
It is not contagious, and cannot be passed on to other people.