Definition: Hemocystosis is a medical condition in which the blood cells, called red blood cells, become too large and clumped together. This can occur in any part of the body, but it's most common in the lungs. Hemocysts are not a normal part of the body, but they can be caused by various conditions such as infection or cancer. If you have hemocystosis, it's important to seek medical attention as soon as possible to prevent further complications.