Cells of the immune system

Barriers

The first line of defence against any invading force is a wall. In the case of the human body, the epithelia are the most extensive mechanical barrier against potential pathogens. The epithelia being discussed include the skin externally and the continuation of the mucous membranes of the respiratory and digestive systems internally.

Although the type of epithelium varies depending on the location (i.e. from keratinized squamous epithelium of the skin to non-keratinized squamous epithelium of the oral cavity to simple mucus secreting columnar epithelium in the stomach), the cells are held together by desmosomes that significantly limits the permeability of the cell membranes.

In addition, the epithelium is also equipped with defensins, which are cationic antimicrobial molecules that facilitate destruction of potential invading pathogens. They are able to carry out this function by depolarizing the membrane of the microbe or lysis of the pathogen’s cell wall. There is also a wide distribution of mucosa-associated lymphatic tissue. These aggregates of secondary lymphoid tissue contain lymphocytes that are able to launch a response to microorganisms in the area. Should there be a breach of this layer, however, additional immune responses will be stimulated.

Pattern recognition receptors

Usually most related microbes have specific detectable properties that are homogenous within the species and not susceptible to genetic variability. Similarly, there are recognizable molecular trends with tissue damage and necrosis. These structures are referred to as pathogen associated molecular patterns (PAMPs) and damage associated molecular patterns (DAMPs), respectively. The detection of these patterns is achieved by a family of pattern recognition receptors (PRRs). They are subdivided into:

• plasma membrane receptors
• endosomal receptors
• cytosolic receptors

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 most commonly discussed pattern recognition receptor is the family of Toll-Like Receptors (TLRs). There are approximately 10 subtypes of toll-like receptors in the mammalian kingdom. Both are found in both the endosomal vesicles and on the plasma membrane.

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). An example of a cytosolic receptor is the nucleotide-binding oligomerization domain-like receptor (NOD-like receptors or NLRs, for short). They are able to detect electrolyte imbalances, by-products of cellular necrosis, and pathogenic substances.

Monocytes

An integral part of the innate immune system is the ability of these cells to engulf and destroy the pathogens. Monocytes and their peripheral derivatives (macrophages) are able to recognize, engulf, and destroy circulating microbes. The monocyte lineage develops from the common myeloid progenitor cells under the influence of granulocyte macrophage colony stimulating factor (GM-CSF). They first form large, uniform cells with a pale nucleus and sine chromatin known as myeloblasts. Monocyte colony stimulating factor (M-CSF) drives the differentiation of myeloblast cells into monoblasts and subsequently to promonocytes. The latter contain a large, indented nucleus with visible nuclei and a basophilic cytoplasm. The cells are considered mature monocytes once there is an increase in the rough endoplasmic reticulum concentration and enlargement of the Golgi complex.

Macrophages

Monocytes that migrate into specific tissue are subsequently referred to as macrophages. They can also be seen in the absence of an infectious or inflammatory process. After macrophages engulf the invading microbes, they are able to present fragments of the pathogen’s protein on their surface. This is referred to as antigen presentation and therefore, macrophages are also considered as antigen presenting cells. In addition, they are considered as potent effector cells as they can be activated by T lymphocytes (in the adaptive immunity) and their ability to kill phagocytosed microbes will be subsequently upregulated. They are also able to phagocytose microbes that have been opsonized in the humoral phase of cellular immunity.

Dendritic cells

Dendritic cells are a subset of leukocytes whose membrane processes resembles those of neuronal dendrites (hence its name). They are quite ubiquitous and can be seen in both primary and secondary lymphoid tissue and throughout the epithelia. Their primary function is to engulf foreign proteins and present them to thymic lymphocytes (T lymphocytes). They are also capable of detecting products of cellular damage as well as pathogens. In response to the presence of these entities, they promote the release of cytokines and the initiation of the inflammatory process. Although they are able to detect the foreign antigen, they lack the ability to phagocytose them.

Natural killer cells

Viral and bacterial coverage is provided innately by the natural killer (NK) cells. They target cells that have been exposed to stressful conditions and have subsequently been irreversibly damaged. They resemble lymphocytes structurally, but are a bit larger in diameter. Their cytoplasm contains numerous azurophilic granules that are filled with hydrolytic and digestive enzymes.

NK cells are devoid of T cell receptors (TCRs) and they do not express immunoglobulins. They are therefore capable of destroying a variety of tumors and infected cells without previous encounter with similar antigen. The key lies in the presence of cluster of differentiation 16 and 56 (CD16 and CD56) markers that are present on the cells. CD16 allows binding to the Fc receptor of IgG, enabling NK cells to lyse IgG opsonized pathogens. This process is referred to as antibody dependent cell mediated toxicity. NK cells are part of a group of TCR negative lymphocytes known as innate lymphoid cells. They allow mounting of an early defence response, perform stress surveillance, and prepare the system for an adaptive response.

Functions

The aim of innate immunity is to initiate an inflammatory response and promote an antiviral defence system. The inflammation is stimulated by the activation of the alternative and lectin complement pathways. This also promotes the release of cytokines that possess vasodilating properties and stimulate cellular recruitment to the site of injury. Viral nucleic acid destruction is promoted by type I interferon production.

Humoral immunity

Humoral immunity deals with extracellular antigen detection and processing. Bone marrow lymphocytes (B lymphocytes) possess immunoglobulins on their cell surfaces. The variable portion of the immunoglobulin (which is the antigen binding fragment [Fab]) has the capacity to bind a wide variety of antigens. However, each B lymphocyte can only bind to a specific set of antigens. Binding of the B lymphocyte to the foreign antigen will result in an increase of antibodies specific to the inciting agent. These antibodies will subsequently opsonize the pathogens and facilitate complement activation.

Helper T lymphocytes also participate in the humoral pathway. After binding with the antigen on the antigen presenting cells, helper T lymphocytes can induce macrophage activity, stimulate the inflammatory cascade by promoting release of cytokines and inciting the proliferation of more B and T lymphocytes. After a B lymphocyte is activated, they often mature into plasma cells that continue to produce immunoglobulins for the antigen they were exposed to. Other B lymphocytes become memory B cells that can readily respond to a recurrent insult by the same or a similar pathogen.

Cell-mediated immunity

Cell-mediated immunity on the other hand handles intracellular pathogens. Once a cell has been exposed to the microbe, the antigen can be reflected on the cell surface (i.e. antigen presenting cells). T lymphocytes are able to bind to major histocompatibility complexes (MHCs) that are found on all cell surfaces.

In fact, there are two subsets of MHCs in humans – type 1 and type 2. MHC I molecules are found on all nucleated cells in the body as well as platelets. They are bound by CD8 positive T lymphocytes. MHC II moieties are normally present on antigen presenting cells and are bound by CD4 positive T lymphocytes. The CD8 positive cells are also referred to as cytotoxic T lymphocytes; they bind to the antigen presenting cell and induce apoptosis. Other antigen presenting cells present in the circulation include the previously discussed dendritic cells, follicular dendritic cells in the lymph nodes, and macrophages. Of note, there are also regulatory T lymphocytes that downregulate the proliferation of more T lymphocytes when necessary.

Functions and nomenclature

While most of the innate immunity mechanisms are cellular, the complement pathway is an enzyme based defence mechanism that opsonizes (tags the cells) and perforates the cell membrane. It works in conglomeration with cells of both the innate and adaptive immunity to eradicate pathogens by initiating the inflammatory process and facilitating phagocytosis. There are three well understood complement pathways. The alternative and lectin pathways are involved in the innate immunity, while the classical pathway is involved in the adaptive immune pathway. Unlike the adaptive immune process, complement does not have antigenic memory and does not evolve with time. The components of complement exist as zymogens. These are proenzymes that require proteolytic cleavage in order to become active. This is important because these enzymes bind indiscriminately and would opsonize healthy cells.

The most difficult part of understanding the complement pathway is getting used to the nomenclature and the rules of complement. The alphanumeric naming system uses a combination of uppercase ‘C’ (complement), a number 1 – 11 to designate the subtype of complement enzymes (the pathways usually focus on numbers 1 to 9) and a lowercase ’a’ or ‘b’ to identify a cleaved fragment of the enzyme. As a general rule, ‘a’ is designated as the smaller of the two complement fragments, while ‘b’ is the larger fragment. The only exception of this rule applies to C2, where the reverse is true (i.e. C2a is larger than C2b).  

As opposed to the C1, C2, etc. designation described above, the alternative pathways refer to the complement enzymes as Factors (i.e. Factor B). Naming of the receptors include adding ‘R’ after the previously describe naming system (i.e. C1R or C5aR). The three complement pathways are initiated using distinct mechanisms. However, they all converge in a common pathway that involves opsonization, aquaporin formation, and cell death.

Classical complement pathway

In the classical pathway, pathogens are tagged by IgM or IgG immunoglobulins. C1 has three different components (q, r, and s). C1q binds to the Fc region of IgM. This activates C1r, which is a serine protease that cleaves C1s. C1s, another proteolytic enzymes goes on to cleave both C4 and C2. The two large fragments of the enzyme breakdown, C4b and C2a, merge to form C4b2a; which is also known as C3 convertase.

C3 convertase has proteolytic activity and subsequently breaks down C3 in C3a and C3b. C3b joins C3 convertase to form C4b2a3b; which is also known as C5 convertase. Of note, C3b also acts as an opsonizing agent and attaches to the cell membrane.

The last step in the classical pathway is the cleavage of C5 into C5a and C5b by C5 convertase. C5b is integral in the formation of the membrane attack complex. Other fragments that have been created during this process (C5a, C3a and C4a in order of decreasing potency) are anaphylatoxins that stimulate basophils, neutrophils, mast cells, and monocytes. This increases the phagocyte recruitment and vascular engorgement as a part of the initiation of the inflammatory response.  Additionally, they promote smooth muscle contraction and increases blood capillary permeability.

Lectin complement pathway

The lectin pathway is very similar to the classical pathway. The primary differences are the initial proenzymes and the binding site. As opposed to the immunoglobulin binding point in the classical pathway, the lectin pathway uses mannose binding lectin (MBL) to form an initial attachment to the microbial surfaces with mannose containing polysaccharides. This is analogous to the C1q binding to the immunoglobulin Fc segment in the classical pathway.

There are two MBL associated serine proteases (MASP-1 and MASP-2) that function similarly to C1r and C1s (respectively), such that MASP-1 is activated by MBL and it subsequently cleaves MASP-2. Activated MASP-2 will then cleave C4 into C4a and C4b. As observed in the classical pathway, C4b will bind to the microbial surface and become attached to C2a after it is also cleaved by MASP-2. The resulting C3 convertase (C4b2a) will cleave C3 to form the C5 convertase (C4b2a3b). The remainder of the pathway is identical from this point onwards.

Alternative complement pathway

Unlike the classical and lectin pathways, the alternative pathway begins with C3. It is cleaved by circulating IgA immunoglobulins or by peptidoglycans (PG) and lipopolysaccharides (LPS) in bacterial and viral cell wall. Therefore, the alternative pathway provides a rapid initial immune response since it does not require pathogen-binding antibodies.

The active form, C3b is attached to the microbial surface and acts as a docking station for Factor B (another serine protease). Factor B is subsequently cleaved by Factor D and the remaining Factor Bb fragment stays bound to C3b to form C3bBb. This complex is capable of operating as both a C3 convertase.  However, this C3 convertase complex is relatively volatile and requires a stabilizing agent in the form of properdin also known as Factor P. The stabilized C3 convertase then cleaves additional C3 zymogens to form C3bBb3b; also known as C5 convertase. C5 convertase cleaves C5 in a similar fashion as observed in the classical pathway and the C5b fragment goes on to initiate the membrane attack complex.

Membrane attack complex

The final product of all the pathways is the C5b enzyme. It binds to C6 and C7. C7 facilitates the attachment of the C5b67 complex to the membrane and the complex is subsequently joined by C8. C8 allows for the complex to be inserted into the phospholipid bilayer. Subsequently, the C5b678 complex induces polymerization of about 20 C9 molecules that form a tubular pathway into the phospholipid bilayer of the microbe known as the membrane attack complex (MAC). The membrane attack complex perforates the cell membrane, resulting in lysis and subsequent death of the invading cell.