The term compromised host means a patient who has an increased risk for life threatening infectious complications as a consequence of a congenital or acquired qualitative or quantitative abnormality in one or more components of the host defense matrix.Infection in an immunocompromised host may be due to anatomical barrier (i.e. skin) damage or due to immunodeficiency disorders which may be primary or acquired.Primary immunodeficiency disorders are inheritable disorders of the immune system, which reflect abnormalities in the development, and /or maturation of the cells of the immune system. Defective development of B cells results in abnormalities in humoral immunity which in turn leads to recurrent pyogenic infection. Defects in the development of T cells cause problems with cellular immunity which in turn lead to opportunistic infections. Examples of acquired immunodeficiency disorders include AIDS, splenectomy and diabetes mellitus. Infection in an immunocompromised host may also be due to neutropenia, which is defined as a decrease in the absolute number of circulating segmented neutrophils and bands in blood. ANC is determined by multiplying the total white blood count by the percentage of segmented and band forms. Neutropenia may be due to disorders of production (congenital or acquired), disorders of neutrophil utilization and turnover, depletion due to infectious diseases, hematological malignancies, and drug induced neutropenia.Lower respiratory tract infections are the most common cause of morbidity and mortality worldwide in immunocompromised patients. The most important lower respiratory tract infection is pneumonia which is an infection or inflammation of the lung that can be caused by a wide range of pathogens. The most common pathogens causing chest infection are Streptococcus pneumonia, Haemophilus influenza, Staphylococcus aureus, and viruses e.g. adenovirus, RSV and influenza virus. Chest infection may be due to atypical pathogens causing atypical pneumonia.“Atypical pathogens” is a vague term used historically to describe causes of community-acquired pneumonia that have clinical and radiographic characteristics differing from those of classic pneumococcal pneumonia. Most authorities currently include Legionella, Mycoplasma pneumoniae, and Chlamydia pneumoniae as the predominant atypical pathogens because of the ever-increasing recognition of their prevalence and lethality. Other implicated pathogens in this category include Bordetella pertussis and Coxiella burnetii. This study was done to diagnose the most common etiological agents causing chest infection in immunocompromised patients using simple indirect immunofluorescence test in addition to sputum culture.This work included 20 neutropenic patients with chest infection and a control group consisting of 10 healthy volunteer of matched age and sex. All patients were subjected to full history and clinical examination, complete blood count with differential counts, chest X-ray, sputum culture, and serological test for atypical bacteria (Legionella pneumophilia group I, Coxiella burnetii, Chlamydia psittaci, Chlamydia pneumoniae, and Mycoplasma pneumoniae) by indirect immunofluorescence (Pneumobact).Among the typical pathogens detected by sputum culture, Staphylococcus aureus was the most common (4 cases) followed by ß hemolytic Streptococci (2 cases). Enterococci (normal flora) and Klebsiella in 1 case each. Candida albicans was found also in 1 case. Escherichia coli and Pseudomonas aeurginosa were detected in 1 case each.Among the atypical pathogens detected by IFA in both patients and controls, Chlamydia pneumoniae was detected in 14 cases, 7 as a sole pathogen, Legionella pneumophilia group I in 9 cases, Coxiella burnetii in 3, Mycoplasma pneumoniae in 3 and Chlamydia psittaci in one case.Eight cases were infected by a single microorganism, 4 by Chlamydia pneumoniae, 1 by Legionella pneumophilia group I with Enterococci (normal flora), 1 by Klebsiella, 1 by Staphylococcal aureus, and 1 by ß hemolytic Streptococci. Seven cases were positive for 2 micro-organisms at a time. Three cases were positive for 3 micro-organisms at a time and 1 case was positive for 4 micro-organisms.Our results showed a significant increase in the incidence of antibodies to atypical pathogens in the patients group as compared to the control group both IgM and IgG levels. In the serological diagnosis of the disease, the only standardized technique is indirect immunofluorescent assay. It is necessary to prove seroconversion in order to confirm a serological diagnosis since high titre may be found in healthy population. A high titre in a single serum sample together with clinical symptoms suggests illness.In our work the commonest atypical bacteria causing IgM titre indicating recent infection was Legionella pneumophilia group I (3 cases). IgG was positive in six patients.In Mycoplasma pneumoniae, IgM is a good marker of acute infection because it persists for only 3-4 months. The most traditional serological method has been the CF whereas ELISA and IFA are now more used. IFA titre of IgG antibodies ≥ 64 suggests recent infection. Although a rising IgG titre is important to diagnose active infection if seroconversion from IgM is not documented, significantly high IgG levels should be respected. In our work IgG incidence against atypical bacteria was significantly higher than in controls.In our work we could detect IgM antibodies to Chlamydia pneumoniae in three cases and IgG in twelve cases of our patients. In Coxiella burnetii, diagnosis is based on serological methods since isolation from clinical sample is difficult. Antibodies against phase II antigen are predominant in the acute phase of the disease. IFA is more sensitive and better for detecting the specific IgM response. IgM antibodies are detected by IFA from the second week till the fourth week of the acute phase of illness. In those cases of acute infection where IgG antibodies are detected at higher titre, it is highly suspected to find IgM as well. Coxiella burnetii IgG antibodies represented in three of our patients.Coinfection was a common finding in our work. The most common association was between Legionella pneumophilia group I and Chlamydia pneumoniae (20%). Mycoplasma pneumoniae & Chlamydia pneumoniae also coexisted in one of our cases (5%).Another coinfection between Chlamydia psittaci and Chlamydia pneumoniae was detected in one case (5%). In our work there was also coinfection with Mycoplasma pneumoniae and Candida albicans in one case as well as the association between Pseudomonas aeruginosa and Chlamydia pneumoniae was found also in one case (5%). Coinfection usually worsens the disease.