Features of the Immune Response to Protozoa
The body's first line of defense in protozoan infections is represented by innate immunity factors. Pathogen recognition occurs via specific pattern-recognition receptors (such as Toll-like receptors, TLRs). Humoral innate defense factors circulate in the blood as soluble receptors, which include mannose-binding lectin and C-reactive protein (an acute-phase protein).
The adaptive humoral response involves the production of antibodies (IgM and IgG). However, their action is strictly limited: they are effective exclusively against extracellular forms of parasites. The main challenge of the humoral response is stage specificity. When the life cycle stage changes (as seen in the malaria parasite Plasmodium), the antigenic composition of the pathogen changes completely. Consequently, the parasite easily escapes previously generated antibodies.
Cell-mediated immunity takes over the destruction of intracellular protozoa. The primary mechanism involves the activation of type 1 T-helper cells (Th1). They produce cytokines, primarily interferon-gamma (IFN-γ), which potently activates macrophages and cytotoxic T lymphocytes (CTLs). Nevertheless, phagocytosis often remains incomplete (a typical example being leishmaniasis), and the pathogen continues to survive inside the phagocyte itself. In clinical practice, features of the cell-mediated response are utilized for diagnostics: delayed-type hypersensitivity (DTH) reactions help detect toxoplasmosis and leishmaniasis.
Mechanisms of Protozoan Immune Evasion
Through evolution, parasites have developed a whole arsenal of strategies to survive and evade immune surveillance:
- Barrier Defense: Formation of dense cysts (typical for Toxoplasma and Acanthamoeba) or the creation of isolated parasitophorous vacuoles (in Cryptosporidium).
- Hidden Forms: Transition into dormant resting stages, typical of the agents causing tertian malaria.
- Safe Localization: Intracellular habitation renders the parasite invisible to antibodies. Additionally, pathogens can colonize immunologically privileged («immune-privileged») sites such as the brain and eyes (frequent targets for Toxoplasma).
- Active Counteraction: Induction of incomplete phagocytosis, along with antigenic variation and immune deviation, which are actively utilized by trypanosomes and plasmodia.
Anti-Helminthic Immunity: The Th2 Pathway
The immune response to multicellular worms (helminths) differs radically from anti-protozoan immunity and is mediated via the Th2 pathway. Key markers of such an infestation include elevated IgE levels, eosinophilia (increased eosinophil count), and mastocytosis (mast cell activation).
CD4+ Th2 helper cells play a leading role in coordinating defense. They exert cytokine regulation over the process:
- IL-4 — Directly stimulates B lymphocytes to synthesize IgE class antibodies.
- IL-5 — Responsible for recruiting eosinophils to the site of inflammation and activating them.
Effector Mechanisms of Helminth Destruction
The primary battle against helminths unfolds in the intestine, as over 99% of all IgE molecules are secreted there. Parasite destruction occurs through the coordinated cooperation of multiple cell types.
Contact Lysis (Role of Eosinophils): Eosinophils locate the helminth and interact with it via the Fc portion of IgE antibodies densely coating the parasite. Upon binding, eosinophils release their toxic granules onto the worm's surface. These granules contain destructive enzymes: major basic protein, peroxidase, and eosinophil cationic protein.
Mechanical Expulsion (Role of Mast Cells): Mast cells also bind to IgE and undergo degranulation, massively releasing histamine. This mediator causes acute smooth muscle spasm in the gut. As a result, peristalsis is drastically enhanced, leading to the mechanical expulsion of helminths from the body. Furthermore, mediators released by mast cells recruit additional eosinophils to the inflammatory focus, amplifying the immune response.