ALC – Absolute Lymphocyte Count
Lymphocytes are a variety of white blood cells present in blood, lymph nodes, spleen, thymus gland, gut wall and bone marrow. Important to the immune system, they produce circulating antibodies and T-lymphocytes, which are primarily responsible for cell-mediated immunity and can differentiate into helper, killer or suppressor cells.
More ALC web definitions via Google here
More via Wikipedia here.