IMHA is a condition of the blood that causes severe anaemia. IMHA causes the body (more specifically the immune system) to attack and destroy its own red blood cells.
Just like people, it is possible for a dog to have an allergic reaction to something such as a bee sting, a medication, or a vaccine.
Lymphoma is a cancer of the immune system. The immune system is active throughout the whole body, and lymphoma can develop in any part of it. Symptoms vary depending on what type it is, where it is, and how aggressive it is.