Innate Immunology: Immune System Overview and Functions

Overview of the Immune System

The Two Branches of the Immune System

• The immune system is divided into two main branches: innate and adaptive.

• Innate Immune System: Acts as the first line of defense, responding quickly (within minutes to hours) and non-specifically to pathogens.

• Adaptive Immune System: Slower response (days to weeks) that is highly specific, recognizing distinct parts of pathogens and establishing immunological memory.

• The innate immune system generates inflammation at sites of infection, while the adaptive system enhances the ongoing innate response.

• Both systems work together to clear pathogens effectively, ensuring survival.

Functions of the Innate Immune System

• Major functions include recognition and destruction of pathogens, as well as the generation of inflammation.

• Inflammation is crucial for recruiting immune cells to sites of infection and facilitating communication between them.

• Without an innate immune system, survival is not possible, highlighting its essential role in host defense.

• The innate immune system utilizes multi-antigen specificity, allowing it to respond to a wide range of pathogens.

• The phrase 'shoot first, ask questions later' encapsulates the innate system's rapid response mechanism.

Communication in the Immune System

Importance of Immune Cell Communication

• Effective immune responses depend on communication between immune cells.

• Questions that immune cells must address include: who is invading, what actions to take, where to go, when to act, and how to coordinate efforts.

• Communication can be direct (via ligand-receptor interactions) or indirect (through soluble factors like cytokines).

• Direct communication involves binding of ligands on one cell to receptors on another, triggering internal signaling cascades.

• Indirect communication allows for long-distance signaling, crucial for coordinating responses across the immune system.

Cytokines and Their Functions

• Cytokines are small, soluble proteins secreted by immune cells that play a key role in signaling.

• Examples include interleukins (e.g., IL-10) and interferons (e.g., IFN-ɣ), which are critical for immune responses.

• Cytokines can induce various actions such as pro-inflammatory responses, anti-inflammatory effects, and differentiation of cell populations.

• They are also important therapeutic targets for various diseases, highlighting their clinical relevance.

• Cytokines help generate inflammation at infection sites, facilitating the recruitment of other immune cells.

Chemokines and Their Role

Overview of Chemokines

• Chemokines act as traffic directors for the immune system, guiding immune cells to sites of infection.

• They can be classified into homeostatic (constitutively expressed) and inflammatory (induced upon activation) chemokines.

• Homeostatic chemokines are critical for the development of immune responses, while inflammatory chemokines aid in cell recruitment during inflammation.

• Chemokines exhibit selective tissue expression, influencing where immune cells migrate.

• The interaction between chemokines and their receptors is often characterized by a 1:1 ratio, although many ligands can interact with a single receptor.

Cytokines vs. Chemokines

• Cytokines: Small, secreted proteins that bind to receptors on target cells, inducing various cellular responses.

• Chemokines: Small proteins secreted in gradients that bind to receptors and induce cell migration.

• Both play crucial roles in the immune response, but their functions differ significantly.

• Cytokines are involved in signaling and differentiation, while chemokines primarily direct cell movement.

• Understanding the distinction between these two types of signaling molecules is essential for grasping immune system dynamics.

Innate Immune Cells

Types of Innate Immune Cells

• The innate immune system is primarily composed of myeloid cells, which include granulocytes (neutrophils, eosinophils, basophils, mast cells) and monocytes/macrophages.

• Neutrophils: The most abundant leukocytes, acting as first responders to infection and major components of pus.

• Eosinophils: Important for defense against helminths and bacteria, residing in tissues and secreting inflammatory mediators.

• Basophils: Rare cells that play a role in parasitic infections and secrete histamine and serotonin.

• Mast Cells: Involved in defense against parasites and allergic reactions, binding IgE and secreting various inflammatory mediators.

Dendritic Cells and Their Role

• Dendritic cells serve as a bridge between the innate and adaptive immune systems.

• Immature dendritic cells reside in tissues and lymphoid organs, maturing upon encountering pathogens.

• Conventional dendritic cells (cDCs) present antigens to T cells, while plasmacytoid dendritic cells (pDCs) specialize in viral detection.

• Dendritic cells secrete inflammatory cytokines, playing a crucial role in initiating adaptive immune responses.

• Their ability to migrate from tissues to lymph nodes is essential for effective antigen presentation.

Accessing Sites of Infection

Mechanisms of Innate Immune Cell Migration

• Innate immune cells gain access to sites of infection through several mechanisms, including chemokine signaling and adhesion molecules.

• Chemokines guide immune cells to the site of infection by creating a concentration gradient.

• Adhesion molecules facilitate the binding of immune cells to the endothelium, allowing them to exit the bloodstream and enter tissues.

• This process is critical for the effective functioning of the innate immune response.

• Understanding these mechanisms is vital for developing therapies targeting immune cell migration.

Inflammatory Cytokines and Chemokines

Overview of Inflammatory Cytokines

• Inflammatory cytokines are signaling molecules that mediate immune responses, particularly during inflammation.

• Under normal conditions, myeloid cells are not recruited to tissues unless there is a pathogen present.

• The balance between homeostatic and inflammatory chemokines is crucial for maintaining immune homeostasis and responding to infections.

Homeostatic vs. Inflammatory Chemokines

• Homeostatic chemokines are constitutively expressed and play roles in the development and maintenance of immune cells.

• Inflammatory chemokines are induced upon activation and are critical for recruiting immune cells to sites of inflammation.

• Key examples of chemokines include CXCL8, CCL3, and CCL11, each attracting specific innate immune cells.

Chemokines and Their Receptors

• CXCL8: Receptors CXCR1 and CXCR2; primarily chemoattracts neutrophils to sites of infection.

• CCL3: Receptors CCR1 and CCR5; attracts macrophages, playing a role in the inflammatory response.

• CCL11: Receptor CCR3; attracts eosinophils and basophils, important in allergic responses.

Adhesion Molecules in Immune Response

• Adhesion molecules such as selectins and integrins are crucial for the migration of innate immune cells into tissues.

• Activated endothelium expresses selectins and ICAMs, facilitating the adhesion and extravasation of leukocytes.

• Integrins on immune cells interact with ICAMs on endothelial cells, promoting tissue infiltration during inflammation.

Mechanisms of Leukocyte Recruitment

Steps of Leukocyte Recruitment

• Step 1: Activation of endothelium by inflammatory cytokines (e.g., TNF-α).

• Step 2: Expression of selectins on activated endothelium, allowing initial rolling of leukocytes.

• Step 3: Chemokine/receptor interactions stabilize adhesion and promote migration into tissues.

• Step 4: ICAMs on endothelium interact with integrins on leukocytes, facilitating firm adhesion.

• Step 5: Extravasation occurs, allowing leukocytes to move into the tissue.

• Step 6: Leukocytes follow chemokine gradients to reach the site of infection or injury.

Leukocyte Adhesion Deficiency (LAD)

• LAD is characterized by defects in the beta-2 subunit of integrins (CD18), leading to impaired leukocyte migration.

• Patients with LAD suffer from recurrent bacterial infections and poor wound healing due to neutrophils' inability to exit the bloodstream.

• Clinical manifestations include gingivitis and the absence of pus formation, indicating ineffective immune response.

Flow Cytometry in Immunology

Introduction to Flow Cytometry

• Flow cytometry is a powerful technique for analyzing the physical and chemical characteristics of cells.

• It allows for the visualization of single cells and can identify rare cell populations within a heterogeneous mixture.

• Cells are labeled with fluorescent antibodies specific to surface proteins, enabling their identification based on fluorescence.

Workflow of Flow Cytometry

• Step 1: Cells are separated based on size (FSC) and complexity (SSC) as they pass through a laser beam.

• Step 2: Populations are gated based on specific surface markers stained with different fluorophores.

• Step 3: Further gating allows for comparison of surface marker expression and identification of rare populations.

Data Visualization in Flow Cytometry

• Histograms provide a simple representation of one parameter, showing the distribution of cell populations.

• Dot plots allow for multiparametric analysis, displaying relationships between two or more markers.

• Understanding how to interpret these plots is critical for analyzing flow cytometry data effectively.

Applications of Flow Cytometry in Immunology

• Flow cytometry is used for fluorescence-activated cell sorting (FACS), allowing for the isolation of specific cell populations.

• It can measure cell proliferation, assess viability, and characterize immune responses.

• FACS can help discover new cell types and understand their roles in health and disease.