# Innate Immunity > See also: > - [[Adaptive Immunity]] > [!abstract]- Innate Immune System 101 > > 1. Border wall (skin) is breached > 2. Macrophages eat and kill > 3. Macrophages call neutrophils > 4. Immune Cells order [[Inflammation & Fevers]] > 5. Reinforcements including [[The Complement System|Complement]] arrive > 6. Complement [[Cytokines|marks]], maims, and kills > 7. Invaders are defeated > > ![[Innate Immune System 101.png|500]] --- ## Physical Barriers The two main physical barriers that assist in defending against pathogens are the **skin layers** and the **mucous membranes** making up the **Skin** is considered to be the first line of defense against infection. - It serves as a tough > Our bodies can be viewed as a giant tube (our digestive system) While our skin (endoderm) are extremely resilient/durable, [[Histology]] ### Mucousal Membranes > See also: > - [[The Gut Microbiome]] **Sebum** is a bacteria-inhibiting mixture of chemicals (including fatty acids) secreted by **sebacuous glands** associated with hair follicles in the skin. --- Skin and mucous membranes are two effective barriers against invading pathogens. All epithelial surfaces secrete antimicrobial substances. Sebum is secreted by sebaceous glands associated with hair follicles in the skin and has an inhibitory effect on bacterial growth on skin surface. Tears and saliva contain lysozyme that kills bacteria by degrading their cell walls. All epithelia produce antimicrobial peptides that kill bacteria, fungi, and enveloped viruses through destabilization of their membranes. Mucus is made by epithelial cells that line the respiratory, gastrointestinal, and urogenital tracts and serves to protect the epithelial or mucosal lining. The epithelial lining of the respiratory tract or respiratory mucosa is continually cleansed of unwanted material, including inhaled infectious microorganisms, by removal of mucus using the action of beating cilia. ## Cell-Mediated Innate Immunity **Toll-like receptors (TLRs)** are able to detect highly conserved antigens known as *pathogen-associated molecular patterns (PAMPS)* - These are primarily expressed by [[Cells of the Immune System|immune cells]] on the front lines of the battle (T-Cells, Neutrophils, Eosinophils, Mast Cells, Macrophages and Epithelial Cells) TLRs are a type of *pattern recognition receptor (PRR)* ## Humoral-Mediated Resistance This component of the innate immune system is where our body’s [[The Human Microbiome|microbiota]] is able to help to fend off pathogens. [[The Complement System]] ### Antimicrobial Peptides - Defensins - Cathelicidins - **Bacteriocins**: toxic peptides released by members of the bacterial microbiota that kill closely related bacteria - Can sometimes increase the bacterial virulence of pathogens - Ex: *colicins* produced by E. coli (Col plasmids) ### Acute-Phase Proteins - Produced during periods of injury/infection - Produced by [[the liver]] - Can assist in prevention of blood loss