Opportunistic Infections in Immunocompromised Hosts

Definition

Organisms harmless/low-pathogenicity in immunocompetent host → significant disease when immune defence impaired. Dominant cause of morbidity/mortality across all immunocompromise types.

Causes of secondary (acquired) immunodeficiency — MORE common than primary

HIV (CD4+ depletion)
Marrow replacement by cancer (leukaemia) — ↓leukocyte progenitors
Immunosuppressive therapy (transplant/autoimmune) — impaired mature lymphocyte function
Irradiation/chemo — ↓marrow precursors all lines
Severe malnutrition — metabolic impairment of lymphocyte function
Splenectomy — ↓phagocytic clearance
Diabetes/DKA — neutrophil dysfunction + angioinvasive fungal risk (mucormycosis)

Defect type → organism pattern (KEY mapping)

Neutropenia/neutrophil dysfunction → bacteria + fungi (Candida, Aspergillus, Pseudomonas, Enterobacteriaceae)
Cell-mediated (T-cell) deficiency → intracellular organisms (mycobacteria, Listeria, Pneumocystis, Cryptococcus, Toxoplasma, CMV, herpesviruses)
Humoral (B-cell)/splenectomy → encapsulated bacteria (pneumococcus, H. influenzae, meningococcus) + enteroviruses
Complement deficiency → Neisseria specifically

Organism list by category (AIDS-context, applies broadly)

Bacterial: TB, MAI, Salmonella, Nocardia. Fungal: Candida, Cryptococcus (meningitis), Histoplasma, Pneumocystis (historically AIDS-defining), Aspergillus, Mucorales. Viral: CMV (retinitis/colitis/oesophagitis), HSV, VZV, JC virus (PML), EBV (lymphoproliferative disease). Protozoal: Toxoplasma (cerebral), Cryptosporidium/Isospora (diarrhoea), Giardia, Entamoeba.

Worked example: Pneumocystis pneumonia

P. jirovecii (human) vs P. carinii (rat). Inhaled. Historically most common AIDS-defining infection.

Morphology: gross = consolidated, dry, grey. Micro = interstitial pneumonitis (thickened alveolar walls, mononuclear infiltrate) + alveolar lumina = pink frothy fluid with organisms (GMS+ cup-shaped/crescentic cysts). Pattern itself (interstitial + frothy exudate, NOT neutrophilic consolidation) = diagnostic clue for opportunistic vs bacterial cause.

Clinical relevance

Defect type → targeted differential (neutropenic fever ≠ T-cell-deficient fever workup). Post-splenectomy → vaccinate against encapsulated organisms specifically (phagocytic clearance loss, not Ab defect). Interstitial+frothy-exudate pneumonia pattern → prompt HIV/immunocompromise workup if not already known.

Definition

Opportunistic infections are those caused by organisms that are typically harmless commensals or of low pathogenicity in an immunocompetent host, but which cause significant disease once the host’s immune defences are impaired. They are the dominant cause of morbidity and mortality across the whole spectrum of immunocompromise — primary (congenital) immunodeficiency, secondary (acquired) immunodeficiency, and iatrogenic immunosuppression alike.

Causes of Susceptibility

Secondary (acquired) immunodeficiency is, as a group, far more common than primary genetic immunodeficiency, and arises from several distinct mechanisms, each of which predisposes to a different pattern of opportunistic organism:

HIV infection — depletion of CD4+ helper T cells (see HIV Infection and AIDS).
Bone marrow replacement by cancer (leukaemia, marrow metastases) — reduced normal leukocyte progenitor development.
Immunosuppressive therapy (for transplant rejection or autoimmune disease) — impaired function of mature lymphocytes and other immune cells (see Transplant Rejection).
Irradiation and cytotoxic chemotherapy for cancer — reduced bone marrow precursors for all leukocyte lines.
Severe acute malnutrition — metabolic derangement impairing lymphocyte maturation and function.
Splenectomy — reduced phagocytic clearance of blood-borne microbes, particularly encapsulated bacteria.
Diabetes mellitus, particularly diabetic ketoacidosis — impaired neutrophil function and a specific predisposition to angioinvasive fungal infection (see Mucormycosis).

The specific type of immune defect predicts the pattern of opportunistic organism encountered, a principle worth applying whenever a clinical scenario names a specific defect:

Neutropenia / neutrophil dysfunction → bacterial and fungal infection (Candida, Aspergillus, Pseudomonas, Enterobacteriaceae).
Cell-mediated (T-cell) immunodeficiency → intracellular pathogens: mycobacteria, Listeria, Pneumocystis, Cryptococcus, Toxoplasma, CMV, herpesviruses, disseminated fungal disease.
Humoral (B-cell/antibody) immunodeficiency or splenectomy → encapsulated bacteria (Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis) and enteroviruses.
Complement deficiency → recurrent Neisseria infection in particular (see Chemical Mediators of Inflammation for the complement regulatory pathway).

Common Opportunistic Organisms by Category

As catalogued in the context of AIDS (the paradigm setting for opportunistic infection, but the same organisms recur across all forms of immunocompromise):

Bacterial — Mycobacterium tuberculosis, Mycobacterium avium-intracellulare (particularly in advanced HIV), Salmonella, Nocardia.
Fungal — Candida (oral/oesophageal/systemic candidiasis), Cryptococcus neoformans (meningitis), Histoplasma capsulatum, Pneumocystis jirovecii (pneumonia — historically the AIDS-defining infection), Aspergillus, Mucorales (mucormycosis, especially in diabetic ketoacidosis).
Viral — cytomegalovirus (retinitis, colitis, oesophagitis), herpes simplex, varicella-zoster, JC virus (progressive multifocal leukoencephalopathy), EBV (associated lymphoproliferative disease).
Protozoal/parasitic — Toxoplasma gondii (cerebral toxoplasmosis), Cryptosporidium and Isospora (chronic diarrhoea), Giardia, Entamoeba histolytica.

Worked Example: Pneumocystis Pneumonia

Pneumocystis jirovecii (the human-infecting species; P. carinii infects rats) causes pneumonia by inhalation, chiefly in neonates and the immunosuppressed. Almost all HIV/AIDS patients develop an opportunistic infection during their disease course, and Pneumocystis pneumonia has historically been the most common such infection and an AIDS-defining illness. Other susceptible groups include transplant/tumour chemotherapy recipients, the malnourished, and patients with agammaglobulinaemia.

Morphology: grossly, affected lung is consolidated, dry and grey. Microscopically: interstitial pneumonitis, with thickened, mononuclear-cell-infiltrated alveolar walls; alveolar lumina filled with pink, frothy fluid containing the organisms, demonstrable with silver stains (GMS) as cup-shaped or crescentic cysts. This pattern — interstitial inflammation with an intra-alveolar frothy exudate, rather than the neutrophil-rich consolidation of bacterial pneumonia — is itself a diagnostic clue pointing toward an opportunistic rather than a routine bacterial cause when seen in an appropriate host.

Clinical Relevance

Matching the type of immune defect to the expected organism category is directly clinically useful: a neutropenic patient with fever needs empirical broad bacterial/fungal cover, whereas a T-cell-deficient patient (advanced HIV, post-transplant) needs a differential that specifically includes Pneumocystis, CMV, and disseminated mycobacterial/fungal disease — the same “fever and infiltrate” presentation is worked up very differently depending on which arm of immunity is compromised.
Post-splenectomy patients require prophylactic vaccination against encapsulated organisms (pneumococcus, H. influenzae, meningococcus) specifically because splenic phagocytic clearance — not T-cell or antibody function per se — is what has been lost, and this is the organism category that clearance mechanism normally handles.
Recognising the interstitial-pneumonitis-with-frothy-alveolar-exudate pattern of Pneumocystis pneumonia, as distinct from lobar consolidation, should prompt HIV testing or evaluation for another underlying immunocompromising condition in a patient who has not already been diagnosed with one.

Diagram 1: Immune Defect → Predicted Organism Pattern (comparative panel)

Draw four boxes side by side, one per type of immune defect, each listing its characteristic organism categories underneath.

Neutropenia/neutrophil dysfunction → bacteria + fungi (Candida, Aspergillus, Pseudomonas, Enterobacteriaceae).
Cell-mediated (T-cell) deficiency → intracellular organisms (mycobacteria, Listeria, Pneumocystis, Cryptococcus, Toxoplasma, CMV, herpesviruses).
Humoral (B-cell/antibody) deficiency or splenectomy → encapsulated bacteria (pneumococcus, H. influenzae, meningococcus) + enteroviruses.
Complement deficiency → Neisseria species specifically.

Labels required

Each box must name its defect type and its organism list together, since the exam-relevant skill is predicting the organism category from a described clinical defect (or vice versa), not memorising the two lists independently.
Splenectomy should be grouped with, but separately noted alongside, humoral deficiency — it produces the same encapsulated-organism vulnerability through a different mechanism (loss of phagocytic filtering, not loss of antibody production).

Errors commonly made

Applying a single generic “opportunistic infection” organism list to every immunocompromised patient, rather than narrowing the differential based on which specific arm of immunity (neutrophil, T-cell, B-cell/humoral, complement) is actually deficient in the case described.
Forgetting that splenectomy causes encapsulated-organism susceptibility through loss of phagocytic clearance, not through an antibody defect, even though the resulting organism pattern looks the same as humoral immunodeficiency.