The immune system has two distinct forms of protection against infection. The cellular immune system is comprised of blood cells (lymphocytes, macrophages and neutrophils). The Humoral Immune system is comprised of protective blood proteins called antibodies (IgG, IgM and IgA) and Complement. Absence or poor function of either or both of these forms of protection can lead to an unusual frequency of infections and / or an unusual severity of infections when they occur. If the immune system is ineffective, infections can occur from organisms that normally do not cause human disease. Typically the most severe immune problems become obvious early in life. Milder forms of immune problems are often missed in childhood. When frequent or severe infections are noted, specific laboratory testing can be done to confirm immune problems. At times, special testing is performed to test the immune system function by looking at the antibody response to injected bacterial or viral extracts (vaccines) or by evaluating the function capability of the blood cells (delayed –type hypersensitivity testing). Allergists (Allergy and Immunology specialists) are trained to methodically assess the function of the immune system however, evaluations of patients with immune systems compromised by HIV infection is usually performed by Infectious Disease specialists.