Introduction

Nephritis is inflammation of the kidneys. It may be caused by a bacterial infection of the kidneys (pyelonephritis (see Section 11, Chapter 149)) or exposure to a toxin. However, it more commonly develops from an abnormal immune reaction, which can occur in two ways: (1) an antibody can attack either the kidney itself or a substance that stimulates an immune reaction (antigen) attached to kidney cells, or (2) an antigen and antibody can combine somewhere else in the body, forming an immune complex, and then attach to cells in the kidney.

Some types of nephritis involve infiltration of kidney tissues by white blood cells and deposits of antibodies. In other types of nephritis, inflammation may consist of tissue swelling or scarring without white blood cells or antibodies. Nephritis can occur anywhere in the kidneys.

Nephritis most often affects the tufts of microscopic blood vessels (glomeruli) with small pores through which blood is filtered. Such inflammation is called glomerulonephritis.

When a glomerulus is damaged, substances not normally filtered out of the bloodstream--such as proteins, red blood cells, and white blood cells--can pass through the glomerulus and enter the fluid that becomes urine. Progressive damage to glomeruli causes urine production to fall and metabolic waste products to build up in the blood. When damage is severe, inflammatory cells and injured glomerular cells accumulate, compressing the capillaries within the glomerulus and interfering with filtration. Scarring may develop, impairing kidney function and reducing urine production. In some cases, tiny blood clots (microthrombi) may form in the small blood vessels, further decreasing kidney function.

There are three major types of glomerulonephritis: nephritic syndrome, nephrotic syndrome, and asymptomatic proteinuria and hematuria syndrome. The three types are not exclusive; a person may have two types simultaneously, or have one type that later develops into another type.

Less commonly, nephritis involves the tubules and the tissues that surround them (tubulointerstitial tissues). Such inflammation is called tubulointerstitial nephritis. A kidney tubule is a microscopic tube that carries fluid and substances filtered from the blood in the glomerulus to the duct that drains urine into the pelvis of the kidney. Tubulointerstitial tissues surround each of the tubules and separate one tubule from another. When inflammation damages the tubules and the tubulointerstitial tissues, the kidneys may become unable to concentrate urine, eliminate (excrete) metabolic waste products from the body, or balance the excretion of sodium and other electrolytes, such as potassium. When the tubules and tubulointerstitial tissues are damaged, kidney failure often develops.

Nephritis may also involve the blood vessels within the kidneys. Inflammation of the blood vessels is called vasculitis.