Innate immunity is your body's first line of defense against invaders. It's like having a team of specialized bouncers ready to kick out unwanted guests. These cellular defenders work together to quickly identify and neutralize threats before they can cause harm.

Each cell type in innate immunity has a unique role. Macrophages are the cleanup crew, neutrophils are rapid responders, and natural killer cells are assassins. Dendritic cells bridge innate and adaptive immunity, while mast cells and eosinophils handle specific threats like parasites and allergens.

Cellular Components of Innate Immunity

Cell types in innate immunity

• Macrophages engulf pathogens through phagocytosis present antigens to T cells produce cytokines for immune regulation (TNF-α, IL-1β)
• Neutrophils quickly respond to infections phagocytose pathogens release granules with antimicrobial substances (myeloperoxidase, defensins)
• Natural Killer (NK) cells recognize and destroy virus-infected cells produce cytokines like IFN-γ to activate other immune cells
• Dendritic cells capture antigens in tissues migrate to lymph nodes present antigens to T cells bridging innate and adaptive immunity
• Mast cells release inflammatory mediators (histamine, leukotrienes) play crucial role in allergic responses and protection against parasites
• Eosinophils defend against parasitic infections (helminths) contribute to allergic reactions through release of cytotoxic granule proteins

Role of macrophages

• Tissue-resident sentinels distributed in various organs (liver, lungs, brain) act as first line of defense against invading pathogens
• Phagocytosis involves engulfment of pathogens and cellular debris formation of phagosomes that fuse with lysosomes for pathogen destruction
• Pattern recognition receptors (PRRs) detect pathogen-associated molecular patterns (PAMPs) activate signaling cascades for immune response
• Activation states include M1 (pro-inflammatory) and M2 (tissue repair) polarization depending on environmental stimuli
• Cytokine production encompasses pro-inflammatory (TNF-α, IL-1β) and immunoregulatory (IL-10, TGF-β) factors shaping immune responses
• Antigen presentation involves processing engulfed pathogens expressing antigens on MHC class II molecules for T cell recognition

Functions of neutrophils

• Rapid recruitment to infection sites guided by chemotaxis extravasate through blood vessel walls following inflammatory signals
• Phagocytosis of pathogens entails engulfment destruction of microorganisms within phagolysosomes
• Degranulation releases antimicrobial proteins and enzymes (myeloperoxidase, defensins, cathepsins) into extracellular space
• Neutrophil extracellular traps (NETs) consist of released DNA and histones trap and kill pathogens outside the cell
• Short lifespan ensures rapid turnover during acute inflammation cleared by macrophages after pathogen elimination
• Oxidative burst produces reactive oxygen species (ROS) kills engulfed pathogens through oxidative damage

Natural killer cells in immunity

• Recognition of abnormal cells detects altered MHC class I expression identifies stress-induced ligands on infected or transformed cells
• Cytotoxic activity involves release of perforin and granzymes induces apoptosis in target cells
• Antibody-dependent cell-mediated cytotoxicity (ADCC) recognizes antibody-coated targets activates NK cells through Fc receptors
• Cytokine production includes IFN-γ for macrophage activation TNF-α for inflammation and cell death
• Killer cell immunoglobulin-like receptors (KIRs) regulate NK cell activity balance activating and inhibitory signals
• Memory-like properties enable enhanced responses to subsequent encounters contribute to trained immunity concept

Dendritic cells as immune links

• Antigen capture and processing involves endocytosis of pathogens processing antigens for presentation to T cells
• Migration to lymphoid organs occurs after maturation upregulation of chemokine receptors for transport to T cell-rich areas
• Antigen presentation utilizes MHC class I and II molecules displays peptides for T cell recognition and activation
• Costimulatory molecule expression includes CD80, CD86 for T cell activation CD40 for interaction with helper T cells
• Cytokine production encompasses IL-12 for Th1 differentiation IL-23 for Th17 differentiation shaping adaptive responses
• Cross-presentation allows presentation of exogenous antigens on MHC class I activates CD8+ T cells against intracellular pathogens
• Tolerance induction involves presentation of self-antigens in steady-state maintains peripheral tolerance preventing autoimmunity