Immunity to microbes: which arm defends against what !

Topic

Each kind of germ is controlled by a different part of the immune system, and each has its own tricks to escape. The table to know.

Updated 2026-11-01

What it is

Where a microbe lives decides which defence works: antibodies, complement and neutrophils clear microbes outside cells; Th1 cells, activated macrophages and CD8 T cells handle microbes inside cells; and Th2 cells, IgE and eosinophils fight worms too big to be eaten. Microbes in turn evade immunity with capsules, antigen changes, hiding inside cells or switching off antigen presentation. Knowing the pairing explains the infection pattern of each immunodeficiency.

Key facts

Innate defences hold the microbe back during the first days; the adaptive response clears it and leaves memory.
Antibodies cannot reach a virus once it is inside a cell; killing the infected cell is then the job of CD8 T cells and NK cells.
NK cells kill cells that hide by lowering MHC class I (the «missing self» signal).
Granulomas wall off microbes that macrophages cannot kill, at the cost of tissue damage (caseous necrosis in tuberculosis).
Much of the disease is caused by the immune response itself, for example the granuloma of tuberculosis or the shock of superantigens.

Microbe type, main defence, evasion and examples

Microbe type	Main defence	How it evades	Examples
Extracellular bacteria	Antibodies (opsonisation, toxin neutralisation), complement, neutrophils; Th17 recruits neutrophils	Polysaccharide capsule that resists phagocytosis; surface antigen variation; IgA proteases; protein A binding the Fc of IgG	Streptococcus pneumoniae, Staphylococcus aureus, Neisseria meningitidis
Intracellular bacteria	Th1 cells and IFN-γ activating macrophages; CD8 T cells; granuloma when not cleared	Block phagosome-lysosome fusion, resist macrophage enzymes, escape to the cytosol	Mycobacterium tuberculosis, Listeria monocytogenes, Salmonella Typhi
Viruses	Type I interferons and NK cells early; CD8 T cells kill infected cells; neutralising antibodies block entry and reinfection	Antigenic drift and shift; less MHC class I on infected cells; latency; infection of immune cells	Influenza virus, HIV, cytomegalovirus
Fungi	Neutrophils (invasive moulds and Candida), Th17 at skin and mucosa, Th1 for intracellular yeasts	Capsule (Cryptococcus), switch between yeast and hyphal forms, survival inside macrophages	Candida albicans, Cryptococcus neoformans, Histoplasma capsulatum
Protozoa	Th1 and IFN-γ, CD8 T cells for intracellular stages; antibodies against blood stages	Antigenic variation of surface proteins; hiding inside host cells	Plasmodium falciparum, Trypanosoma cruzi, Leishmania
Helminths	Th2 cytokines (IL-4, IL-5, IL-13), IgE, eosinophils and mast cells; more mucus and gut motility	Large size; thick tegument; coating with host molecules; long chronic infection with dampened responses	Ascaris lumbricoides, Schistosoma

Clinical notes

Antibody, complement or spleen defects: recurrent infections by encapsulated bacteria (pneumococcus, Haemophilus, meningococcus).
Th1/IFN-γ defects, HIV or anti-TNF therapy: tuberculosis, non-tuberculous mycobacteria and Salmonella.
Neutropenia or neutrophil defects: Staphylococcus aureus, gram-negative bacteria and Aspergillus.
Eosinophilia with high IgE points to helminths; in Colombia think of Ascaris, Strongyloides and Toxocara.
Colombian context: tuberculosis, malaria (P. falciparum and P. vivax), Chagas disease and leishmaniasis remain common, so these defence/evasion pairs show up in daily practice.

Test yourself

Think of the answer, then tap the question to see it.

Which immune arm is most important against Mycobacterium tuberculosis?

Th1 cells and IFN-γ that activate macrophages (with TNF to maintain the granuloma).

How does a polysaccharide capsule help a bacterium evade immunity?

It resists phagocytosis and complement deposition; opsonising antibodies are needed to clear it.

Why do NK cells kill cells infected by viruses that lower MHC class I?

The loss of class I removes an inhibitory signal for NK cells (missing self), so the cell is killed.

Which cells and antibody fight helminths?

Th2 cells drive IgE and eosinophils (plus mast cells); eosinophils kill IgE-coated worms by ADCC.

What are two examples of antigenic variation used by microbes to escape antibodies?

Influenza drift and shift of its surface proteins; surface protein variation in trypanosomes or Plasmodium (also gonococcal pili).

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Educational content on the immune system; not a treatment or individual vaccination guide and no substitute for a health professional's advice. No doses are given.

Sources: 21.5 The Immune Response against Pathogens (OpenStax (Anatomy and Physiology 2e), 2022); 21.2 Barrier Defenses and the Innate Immune Response (OpenStax (Anatomy and Physiology 2e), 2022); 18.3 T Lymphocytes and Cellular Immunity (OpenStax (Microbiology), 2016).

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Representative model, for educational purposes only.