Viral entry into host cells:
Attachment: The first step is attachment, during which the virus binds to specific receptors on the surface of the host cell. These receptors are often proteins that the virus recognizes and uses as entry points. The binding between the virus and the receptor is highly specific and determines the cell types that the virus can infect.
Entry/penetration/internalization: After attachment, the virus gains entry into the host cell. Different viruses employ various methods for entry, including direct fusion with the host cell membrane, endocytosis (where the cell engulfs the virus into a membrane-bound vesicle called an endosome), or membrane penetration (where the virus injects its genetic material directly into the host cell).
Uncoating: Following penetration, the viral capsid or envelope is typically disassembled or uncoated, releasing the viral genome (DNA or RNA) into the host cell. This step allows the viral genome to be released and become accessible for replication and transcription.
Replication and Assembly: The viral genetic material takes control of the host cell’s machinery and forces it to replicate and transcribe viral components. Viral proteins are synthesized, and new viral genomes are produced. These components then assemble to form new viral particles.
Release: The newly assembled viral particles are released from the host cell. This release can occur through cell lysis, where the cell is destroyed, or through budding, where the virus acquires an envelope from the host cell’s membrane as it exits.
Mechanisms Viruses Employ to Evade the Host Immune System:
Viruses have evolved several strategies to evade the host immune system, allowing them to establish and maintain infection within the host. Common mechanisms include:
- Antigenic Variation: Some viruses, particularly RNA viruses like influenza and HIV, can mutate rapidly leading to changes in viral surface proteins, making it difficult for the host immune system to recognize and target the virus effectively. As a result, the virus can continuously evade immune responses and persist in the host.
- Immune Evasion Proteins: Many viruses produce proteins that specifically target and inhibit various components of the host immune system. For example, some viruses produce proteins that interfere with the host’s ability to produce interferons, which are important antiviral signaling molecules. By dampening the production or function of these molecules, the virus can escape immune detection.
- Latency and Persistent Infections: Certain viruses, like herpesviruses and retroviruses (e.g., HIV), can establish latent infections. During latency, the viral genome integrates into the host cell’s DNA and remains dormant, evading immune detection. Later, under certain conditions or stimuli, the virus can reactivate and start replicating, leading to recurrent infections.
- Inhibition of Apoptosis: Apoptosis is a programmed cell death mechanism used by infected cells to limit viral replication and spread. Some viruses produce proteins that inhibit apoptosis, allowing infected cells to survive and produce more viruses without triggering the immune response.
- Immune Mimicry: Some viruses produce proteins that mimic host molecules, tricking the immune system into tolerating them. This prevents the immune system from recognizing the virus as a foreign invader and mounting an effective response.
- Hiding in Immune-Privileged Sites: Certain viruses can establish infection in immune-privileged sites, such as the central nervous system or the placenta. These sites have reduced immune surveillance, allowing the virus to persist without immediate immune clearance.
- Intracellular hiding: Viruses can localize themselves within specific cellular compartments that are less accessible to the immune system. For instance, some viruses reside within the nucleus or in vesicles that prevent immune detection and attack.
- Modulation of host immune response: Viruses may modulate the host immune response to their advantage. They can interfere with signaling pathways involved in immune activation or dampen the immune response through the production of viral proteins that inhibit immune cell functions.
- Immune system interference: Viruses can interfere with various components of the host immune system to evade detection and elimination. They may inhibit the production of interferons, which are important antiviral signaling molecules, or suppress the activation and function of immune cells, such as T cells and natural killer cells.