This is a more “primitive” component of the immune system: provides “first-line” defense
The cardinal signs of inflammaton (tumor = swelling, rubor = redness, calor = heat, dolor = pain) are manifestations of the innate immune system response
Recognition of organisms via pattern-recognition receptors (PRR’s): these recognize “non-self” antigens from the invading organisms
Innate immune system acts immediately to respond to an infection/exposure
Innate immune system activates and instructs the actions of the adaptive immune system
Innate immune system regulates inflammation, maintaining immunologic homeostasis
Components
Non-Cellular Components of the Innate Immune System
Physical Barriers to Invasion: tight junctions in the skin, epithelial and mucous membrane surfaces, mucus itself)
Cell Surface Receptors that Detect Microorganisms and Initiate a Defensive Response: Toll‐like receptors, etc
Epithelial and phagocytic cell enzymes (eg, lysozyme) which selectively disrupt microbial cell membranes
Inflammation‐related serum proteins (eg, complement which also disrupts microbial membranes, C‐reactive protein, lectins such as mannose‐binding lectin and ficolins, hemopexin and haptoglobin, which deny iron to invasive microbes)
Surface and phagocyte granule antimicrobial peptides (defensins, cathelicidin, and many more)
Cellular Components of the Innate Immune System
Phagocytes: myelocytic neutrophils, monocytes, and macrophages
Inflamamtory Cells Which Release Cytokines and Other Mediators: macrophages, mast cells, lymphoid natural‐killer (NK) cells
Adaptive Immune System
Function: immune system that evolves over lifetime of exposure to micro-organisms in the environment
This is a more “modern” component of the immune system: provides more sophisticated, but delayed defense
Antigen‐specific responses to a foreign antigen or pathogen: typically requires days-weeks to fully respond
Cells exposed to microbial antigens are selected for clonal expansion
Immunologic Priming: initial contact with an antigen -> subsequent antigen exposure leads to more rapid and vigorous immune responses (ie: immunologic memory)
Components: both are derived from a common progenitor cell
T-Lymphocytes: principal effectors of antigen-mediated cellular immunity
Cell-mediated immunity is responsible for defense against pathogens that replicate intracellularly (viruses, mycobacteria, and some bacteria), cells which exhibit aberrant differentiation (neoplasms), and allogenic cells (graft rejection)
T-cell responses are initiated by antigen-presenting cells: dendritic cells and monocytes/macrophages (note: monocytes may also differentiate into dendritic cells)
T-cell clonal expansion subsequently occurs
T cells are categorized on the basis of the pattern of cytokines that they secrete, which results in either humoral immune response (TH2) or cell‐mediated immune response (TH1)
B-Lymphocytes: principal effectors of humoral immunity
B-cells are activated after T-cells -> antibodies are produced when B cells encounter antigens and subsequently undergo activation, proliferation, and differentiation
Neonates rely predominantly on maternal IgG (passed through the placenta) and maternal IgA (transferred through colostrum and breast milk)
After infancy, the child’s immunoglobulins gradually increase as levels of maternal antibodies wane
Location
T cell precursors arise from hematopoietic stem cells and home to the thymus for maturation
Mature T-cells, B-cells, monocytes, and dendritic cells enter the circulation -> home to peripheral lymphoid organs (lymph nodes, spleen), mucosal surface‐associated lymphoid (MALT) tissue (gut, genitourinary, and respiratory tracts), skin, and mucous membranes
