Innate Immune System Components to Know for Immunobiology

The innate immune system is our body's first line of defense against pathogens. It includes physical and chemical barriers, specialized cells, and signaling molecules that work together to detect and eliminate threats, laying the groundwork for a robust immune response.

1. Physical barriers (skin, mucous membranes)

   • The skin acts as a primary physical barrier, preventing pathogen entry.
   • Mucous membranes line body cavities and secrete mucus to trap pathogens.
   • Both barriers are reinforced by tight junctions between cells, limiting permeability.
   • The presence of normal flora on skin and mucous membranes provides competitive protection against pathogens.

2. Chemical barriers (pH, enzymes, antimicrobial peptides)

   • The acidic pH of the skin and stomach inhibits pathogen growth.
   • Enzymes like lysozyme in saliva and tears break down bacterial cell walls.
   • Antimicrobial peptides (AMPs) disrupt microbial membranes, providing a rapid response to infection.
   • Secretions such as sweat and saliva contain various antimicrobial substances that neutralize pathogens.

3. Complement system

   • A group of proteins that enhance the ability of antibodies and phagocytic cells to clear pathogens.
   • Activation can occur via three pathways: classical, alternative, and lectin.
   • Complement proteins can opsonize pathogens, making them easier for phagocytes to engulf.
   • The system also promotes inflammation and can directly lyse pathogens through the formation of the membrane attack complex (MAC).

4. Phagocytes (neutrophils, macrophages)

   • Neutrophils are the first responders to infection, rapidly migrating to sites of inflammation.
   • Macrophages are derived from monocytes and play a key role in both phagocytosis and antigen presentation.
   • Both cell types utilize various receptors to recognize and engulf pathogens.
   • They release cytokines to recruit additional immune cells and orchestrate the immune response.

5. Natural killer (NK) cells

   • NK cells are a type of lymphocyte that can recognize and kill virus-infected or tumor cells without prior sensitization.
   • They release cytotoxic granules containing perforin and granzymes to induce apoptosis in target cells.
   • NK cells also produce cytokines that enhance the immune response and activate other immune cells.
   • They play a crucial role in the early defense against viral infections.

6. Dendritic cells

   • Dendritic cells are antigen-presenting cells that capture and process antigens from pathogens.
   • They migrate to lymph nodes to present antigens to T cells, bridging innate and adaptive immunity.
   • Dendritic cells are essential for initiating and shaping the adaptive immune response.
   • They express various pattern recognition receptors (PRRs) to detect pathogens and activate immune responses.

7. Pattern recognition receptors (PRRs)

   • PRRs are proteins on immune cells that recognize pathogen-associated molecular patterns (PAMPs).
   • They play a critical role in the early detection of infections and activation of innate immune responses.
   • Types of PRRs include Toll-like receptors (TLRs) and NOD-like receptors (NLRs).
   • Activation of PRRs leads to the production of cytokines and the recruitment of immune cells.

8. Cytokines and chemokines

   • Cytokines are signaling molecules that mediate and regulate immunity, inflammation, and hematopoiesis.
   • Chemokines are a subset of cytokines that specifically attract immune cells to sites of infection or injury.
   • They play a vital role in cell communication and coordination of the immune response.
   • Dysregulation of cytokine production can lead to inflammatory diseases or immune disorders.

9. Acute phase proteins

   • Acute phase proteins are produced by the liver in response to inflammation and infection.
   • Examples include C-reactive protein (CRP) and serum amyloid A, which help opsonize pathogens and activate complement.
   • They serve as biomarkers for inflammation and can indicate the presence of infection or tissue damage.
   • Acute phase proteins also modulate the immune response and promote tissue repair.

10. Inflammation

    • Inflammation is a protective response to tissue injury or infection characterized by redness, heat, swelling, and pain.
    • It involves the recruitment of immune cells, increased blood flow, and the release of inflammatory mediators.
    • Inflammation helps contain and eliminate pathogens while promoting healing.
    • Chronic inflammation can lead to tissue damage and is associated with various diseases, including autoimmune disorders.