How can you tell if you have a immunodeficiency from a cbc?

There are a variety of immunodeficiencies. Some are present at birth and are called primary immunodeficiencies, other arise from diseases like HIV, auto immune disorders and nutritional problems. An Immunodeficiency panel would contain not only a CBC, but a differential analysis of cell subtypes, a blood smear, studies of specific B and T cells, quantitative serum Antibody tests, etc.

A CBC is not sufficient investigation for immunodeficiency. CBC may be abnormal in immune deficiency, but CBC by itself is unlikely to provide diagnostic information.