Viral Immunity and Immunology

Immune Response to Viral Infections

• Generalized immunosuppression can be caused by viruses such as mumps, measles, EBV, CMV, and HIV
• Antigenic variation is a mechanism used by viruses, such as influenza virus, to evade the immune system

Immune Response to Bacterial Infections

• Innate immunity involves complement activation, phagocytosis, and the inflammatory response
• Acquired immune responses involve humoral mechanisms (antibodies) and cell-mediated immune responses
• Extracellular bacteria replicate outside cells and induce inflammation, often with pus formation
• Genetic variation can lead to the emergence of new strains or variants with altered virulence or drug resistance profiles

Mechanisms of Immune Evasion

• Capsular polysaccharides can prevent effective phagocytosis (e.g., S. pneumoniae)
• Genetic variation of surface antigens can occur (e.g., Neisseria sp., E. coli)
• Inhibition of complement activation can occur (many bacteria)
• Production of IgA proteases can occur (e.g., Neisseria sp., S. pyogenes)

Superantigens

• Superantigens can activate a large proportion of T cells, leading to massive cytokine production and injurious effects

Immune Response to Fungal Infections

• Fungi can survive and replicate in cells, escaping antibody and complement
• Induction of chronic infections can occur

Immune Response to Protozoan Diseases

• Both humoral and cell-mediated immune responses are involved in immunity to protozoan infections
• Humoral antibody is effective against blood-borne stages of the protozoan life-cycle
• Cell-mediated immunity is necessary once protozoans have infected host cells
• Macrophages must be activated by T-cells to enhance killing mechanisms
• Larger protozoa utilize antibody-mediated responses

Immune Response to Malaria

• Sporozoites are prevented from infecting liver cells through antibody-mediated response
• Parasites in liver cells are mediated by CD4+ and CD8+ T-cells
• Parasites inside RBCs are destroyed through Th1 cells producing IL-2 and IFN-γ
• Th2 cells drive antibody production, blocking invasion of new RBCs and activating complement and destroying infected RBCs

Immune Escape Mechanisms of Protozoans

• Protozoans escape the immune response through mechanisms such as:
  • Constantly changing their surface antigenic coat (e.g., Trypanosoma brucei)
  • Sloughing off their glycoprotein coat after antibody binding
  • Absorbing host molecules to their surfaces (e.g., Schistosomes)

Premunition and Concomitant Immunity

• Premunition is a state of partial immunity acquired after a previous infection, where the individual remains persistently infected but asymptomatic or experiences milder symptoms
• Concomitant immunity (CI) refers to a state of effective anti-larval immunity coupled with persistent adult infection

Laboratory Tests

• Laboratory tests can include bacteriological, parasitological, immunological, serological, and molecular diagnosis