Humoral immunity deals with extracellular antigen detection and processing. It is further subdivided into humoral and cellular components. It has a higher specificity and a lower response time than innate immunity. While innate immunity is present since birth, adaptive immunity is dependent on antigenic exposure. Viral nucleic acid destruction is promoted by type I interferon production. This also promotes the release of cytokines that possess vasodilating properties and stimulate cellular recruitment to the site of injury. The inflammation is stimulated by the activation of the alternative and lectin complement pathways. The aim of innate immunity is to initiate an inflammatory response and promote an antiviral defence system. They allow mounting of an early defence response, perform stress surveillance, and prepare the system for an adaptive response. NK cells are part of a group of TCR negative lymphocytes known as innate lymphoid cells. This process is referred to as antibody dependent cell mediated toxicity. CD16 allows binding to the Fc receptor of IgG, enabling NK cells to lyse IgG opsonized pathogens. The key lies in the presence of cluster of differentiation 16 and 56 (CD16 and CD56) markers that are present on the cells. They are therefore capable of destroying a variety of tumors and infected cells without previous encounter with similar antigen. NK cells are devoid of T cell receptors (TCRs) and they do not express immunoglobulins. Their cytoplasm contains numerous azurophilic granules that are filled with hydrolytic and digestive enzymes. They resemble lymphocytes structurally, but are a bit larger in diameter. They target cells that have been exposed to stressful conditions and have subsequently been irreversibly damaged. Viral and bacterial coverage is provided innately by the natural killer (NK) cells. Interdigitating dendritic cells (histological slide) Natural killer cells The cells are considered mature monocytes once there is an increase in the rough endoplasmic reticulum concentration and enlargement of the Golgi complex. The latter contain a large, indented nucleus with visible nuclei and a basophilic cytoplasm. Monocyte colony stimulating factor ( M-CSF) drives the differentiation of myeloblast cells into monoblasts and subsequently to promonocytes. They first form large, uniform cells with a pale nucleus and sine chromatin known as myeloblasts. The monocyte lineage develops from the common myeloid progenitor cells under the influence of granulocyte macrophage colony stimulating factor ( GM-CSF). Monocytes and their peripheral derivatives ( macrophages) are able to recognize, engulf, and destroy circulating microbes. MonocytesĪn integral part of the innate immune system is the ability of these cells to engulf and destroy the pathogens. They are able to detect electrolyte imbalances, by-products of cellular necrosis, and pathogenic substances. An example of a cytosolic receptor is the nucleotide-binding oligomerization domain-like receptor (NOD-like receptors or NLRs, for short). Their primary role is to activate transcription factors including nuclear factor κB (NF-κB which upregulates the synthesis and release of cytokines) and interferon regulatory factors (IRFs which promotes the production of cytokines specific for viral infections). Both are found in both the endosomal vesicles and on the plasma membrane. There are approximately 10 subtypes of toll-like receptors in the mammalian kingdom. The most commonly discussed pattern recognition receptor is the family of Toll-Like Receptors (TLRs). The plasma membrane receptors detect extracellular microbes, the endosomal receptors are able to bind to phagocytosed pathogens, and the cytosolic receptors detect intracellular pathogens. The detection of these patterns is achieved by a family of pattern recognition receptors (PRRs). They are subdivided into: These structures are referred to as pathogen associated molecular patterns (PAMPs) and damage associated molecular patterns (DAMPs), respectively. Similarly, there are recognizable molecular trends with tissue damage and necrosis. Usually most related microbes have specific detectable properties that are homogenous within the species and not susceptible to genetic variability.

Lymphocytes (histological slide) Pattern recognition receptors

Immune system cells