IMMUNE DEFICIENCY SYNDROMES AS MODELS FOR STUDY OF IMMUNE DEFENSES AGAINST INFECTIONS IN HUMANS
Most of our information about the immune system in humans has been learned from the study of patients with immunodeficiency diseases . The most characteris-tic clinical feature of immunocompromised patients are the repeated or chronic infections, often caused by opportunistic agents. Some characteristic associations between specific types of infections and generic types of immune deficiency provide the best glimpse of the physiological role of the different components of the immune system:
1. Patients with antibody deficiencies and conserved cell-mediated immunity suffer from repeated and chronic infections with pyogenic bacteria.
2. Patients with primary deficiencies of cell-mediated immunity usually suffer from chronic or recurrent fungal, parasitic, and viral infections.
3. Neutrophil deficiencies are usually associated with bacterial infections caused by common organisms of low virulence, usually kept in check through nonim-mune phagocytosis.
4. Isolated complement component deficiencies are also associated with bacterial in-fections, most frequently involving Neisseria gonorrhoeae and Neisseriameningitidis, whose elimination appears to require the activation of the mem-brane attack complex.