Overview of the Immune System; Part One

The overall function of the immune system is to prevent or limit infection. It is essential for survival. Multiple organ systems, cells, and proteins are involved in the immune response. It is the most complex system that the human body has. The immune system is differentiated into two directions. Innate or non-specific immunity or Acquired (specific) immunity.

The Innate immune system consists of many components. The skin acts as a mechanical barrier and is typically the first line of defense against foreign substances. Mucous membranes consist of the bodies normal microbiota which compete with invading microbes. The mucous membranes are also lined with mucous and cilia which act in an elevator type motion to push foreign substances away. Physiological barriers such as temperature, pH and the complement system. The more acidic environment that a lower pH offers disrupts bacterial growth. Antimicrobial proteins and peptides are present in different epithelial locations in the body. Lysozymes are present in the tears and saliva and cleave the peptidoglycan cell wall present in bacteria. Secretory phospholipase A2 is present in the gut and can enter the bacterial cell and hydrolyze lipids in the cell membrane. Lectins target gram positive bacteria and forms pores in the membranes. Defensins integrate into the lipid and form pores which causes loss of membrane integrity. These defensins are present in PMNs (neutrophils) and lamellar bodies in the gut. Cathelicidins are present in neutrophils and macrophages in the lungs and intestines and distrupt membranes. Histatins are constitutively produced by the glands in the oral cavity and are active against pathogenic fungi.  Inflammation plays a huge role in the Innate immune system. Inflammation induces vasodilation and increase in capillary permeability causing an influx of immune cells like PMNs and macrophages. Inflammation can be observed by the four cardinal signs; rumor (redness), tumor (swelling), color (heat), and dolor (pain). The innate immune response is a rapid response.

Innate Immunity

The complement system recognizes features of microbial surfaces and marks them for destruction by coating them with C3b. There are three distinct pathways; the classical pathway, the lectin pathway, and the alternative pathway. All pathways generate a C3 convertase which cleaves C3, leaving C3b bound to the microbial surface and releasing C3a. In the classical pathway the activated C1s cleaves C4 to C4a and C4b which binds to the microbial surface. C4b then binds C2, which is cleaved by C1s to C2a and C2b forming the C4b2b complex. C4b2b on the microbial surface is an active C3 convertase which cleaves C3 to C3a and C3b. This results in opsonization of the bacterial surface by C3b. The C4b2b3b complex is an active C5 convertase leading to the development of the membrane-attack complex. Each complement component (C4a/b, C2a/b, C3a/b) have different functions, but that is another discussion for another time. The lectin pathway of complement activation is when mannose-binding lectin (MBL) and ficolins recognize and bind to carbohydrates on the pathogen surface. Ficolins are similar to MBLs, but have a different carbohydrate binding domain. MBLs bind with high affinity to mannose and fucose residues. Conversely ficolins bind oligosaccharides containing acetylated sugars. When MBL binds to a pathogen surface MBL-associated serine protease (MASP)-2 is activated and cleaves C4 and C2 similar to the classical pathway. The alternative pathway is an amplification loop for C3b formation that is accelerated by properdin (factor P) in the presence of pathogens. Properdin stabilizes the C3bBb complex. C3 undergoes spontaneous hydrolysis to C3(H20) which binds to factor B, allowing it to be cleaved by factor D into Ba and Bb. The C3(H20)Bb complex is essentially a C3 convertase which cleaves more C3 into C3a and C3b. C3b molecules result in opsonization of bacterial surfaces. Its important to recognize that all pathways lead to generation of a C5 convertase. C4b2a4b in the classical pathway, C4b2a3b in the lectin pathway, and C3b2Bb in the alternative pathway. C5 is cleaved into C5a/b that initiates the assembly of the terminal complement components. These are the terminal complement components that form the membrane-attack complex.


The membrane attack complex consists of an assembly of C6, C7, and C8. This complex undergoes a conformational change that results in polymerization of C9 which generates a large pore in the cell membrane. Host cells contain CD59 which prevents the assembly of the C9 molecules preventing the formation of the membrane-attack complex.

C3a, C4a, and C5a are unique in that these complement components are called anaphylatoxics. They initiate a local inflammatory response when systemic injection of these molecules occurs. They induce smooth muscle cell contraction and increased vascular permeability. They induce adhesion molecules and activate mast cells that invade and populate submucosal tissues to release inflammatory mediators such as histamine and TNF-a.

The Acquired or adaptive immune system is all about specificity. The Humoral branch of the acquired immune system is executed by the B lymphocytes that produce antibodies to specific antigens. The cell-mediated branch consists of antigen presenting cells (APC) such as the dendritic cells processing foreign substances and presenting proteins of those substances as antigens through the major histocompatibility complex (MHC) to CD8 T lymphocytes. These are cytotoxic T-cells that kill these foreign antigens. The acquired immune response is a slow response because it takes the body time to produce antibodies. An important aspect of the adaptive response is memory. Once antibodies have been produced to an antigen, these responses last and the time it takes to produce an antibody on subsequent exposures is rapidly decreased.

These two different systems work in conjunction to produce an adequate and sustained response. When foreign antigens are processed and expressed on the surface of APCs as MHC peptides, pro-inflammatory cytokines such as IL-12p70, IL-18, and IFN-a are secreted. These attract NK cells which primarily attack viruses as well as PMNs and macrophages that phagocytize these antigen peptides to destroy them. Adaptive immunity is also started with dendritic cells that also undergo antigen uptake and processing. This is also called the maturation signal. This signal is augmented by IFN-y and TNF-a secreted by macrophages and NK cells. These dendritic cells either present the antigen to B lymphocytes which are the antibody producers or they present the antigen to CD4/CD8 T-cell lymphocytes.

There are multiple classes of antibodies. IgD is typically expressed on B-cell lymphocytes during differentiation with IgM. IgD is also present in the serum in low concentrations. IgM is a pentamer and the largest immunoglobulin. It is the first antibody that is produced in the immune response. IgA is in high concentration in the mucosal linings, saliva, and tears. Typically part of first line defenses. IgG is present in high concentrations in the serum. IgG is unique in that it can cross the placenta. IgE is involved in allergic reactions. It binds to mast cells and basophils causing degranulation.



Non-Malignant Leukocyte Disorders

Non-Malignant simply means that it is localized to the leukocytes. Leukocytes are another name for the white blood cells, more specifically in the case of these disorders, the granulocytes. These disorders are fairly uncommon and are inherited. The following are ones that are found to distinct morphological features and affect the granulocyte functionality.

Alder Reilly Anomaly

Alder Reilly Anomaly is a recessive trait defect that causes incomplete degranulation of mucopolysaccharides. Large, darkly staining metachromatic cytoplasmic granules which can be seen and are partially digested mucopolysaccharides. These granules are characteristically referred to as Alder Reilly bodies. These can sometimes resemble toxic granulation, but it is important to note that in toxic granulation neutropenia, dohle bodies, and a left shift is seen. In Alder Reilly Anomaly none of those are present. Its also important to mention that the functionality of the granulocytes is not impaired.

Alder Reilly

Pelger Huet Anomaly

Pelger Huet Anomaly is an autosomal dominant syndrome characterized by decreased nuclear segmentation. This is caused by a mutation in the Lamin B receptor gene. Lamin B is an inner nuclear membrane protein that plays a role in normal leukocyte nuclear shape change during maturation. Morphological changes include hyposegmented neutrophils or neutrophil lobes connected by a thin nuclear filament. Pseudo or acquired PHA can be observed in the granulocytes in individuals with MDS, AML, or chronic myeloproliferative neoplasms.

Pelger Huet

Chediak Higashi Syndrome

Chediak Higashi Syndrome is characterized by an abnormal fusion of granules. These present as large and are dysfunctional. This is caused by a mutation in the LYST, or CHS1 gene that encodes for proteins involved in vesicle fusion or fission. The mutated protein causes loss of lysosomal movement and loss of phagocytosis. Thus leaving the individual susceptible to an increased number of infections without the innate immune system to fight them off. One of the characteristic findings is neutropenia.


May-Hegglin Anomaly

May-Hegglin Anomaly is a rare autosomal dominant platelet disorder that is characterized by variable thrombocytopenia, giant platelets, and dohle bodie like inclusions in the granulocytes. MHA is caused by a mutation in the MYH9 gene that causes a dysfunctional and disarray production of myosin heavy chains type IIa which affects the megakaryocytic maturation process as well as platelet fragmentation. Though most cases are clinically asymptomatic, the individual may present with mild bleeding tendencies.


Chronic Granulomatous Disease

In CGD, mutations in proteins that make up the NADPH oxidase complex. The mutations lead to failure of the phagocytes to generate the oxygen-dependent respiratory burst following phagocytosis. Normal phagocytosis of a microorganism leads to phosphorylation of cytosolic P47 and P67. Antibacterial neutrophil elastase and cathepsin G from the primary granules and cytochrome complex gp91 and gp22 from the secondary granules migrate to the phagolysosome. NADPH oxidase is formed when P47 and P67 combine with P40, RAC2, and the cytochrome complex. Majority of cases of CGD is due to mutations in P47 or gp91.

Leukocyte Adhesion Disorders

Normal recruitment of leukocytes to a site of inflammation involves capture of leukocytes from peripheral blood, followed by a process known as rolling along a vessel wall. Rolling involves binding of integrins to endothelial cell receptors which is high-affinity which ultimately leads to diapedesis of leukocytes into tissues from peripheral blood. With Leukocyte Adhesion disorders there are mutations that result in the inability of neutrophils and monocytes to adhere to endothelial cells, and the consequence is potentially fatal bacterial infections.

Leukocyte Adhesion Disorder I is caused by a mutation in the genes responsible for B2 integrin subunits. This leads to a decreased amount of the truncated form of the B2 integrin which is essential for endothelial cell adhesion. Patients typically present with neutrophilia, lymphadenopathy, splenomegaly, and characteristic skin lesions.

Leukocyte Adhesion Disorder II is caused by a mutation in the SLC35C1 gene. This gene encodes for a fucose transporter that moves fucose from the endoplasmic reticulum to the Golgi region. Fucose is needed for the synthesis of selectin ligands. The defective fucose transporter leads to the inability to produce functional selectins and causes defective leukocyte recruitment and reoccurring infections. LADII is much more rare than LADI. Clinical presentation is growth retardation, coarse facial features, and other physical deformities.

Leukocyte Adhesion Disorder III is even more rare than LADII and is caused by a mutation in the Kindlin-3 gene. The mutations impair leukocyte rolling and activation of B integrin. With LADIII there is also decreased platelet integrin GPIIbIIIa resulting in bleeding similar to that of Glanzmann Thrombasthenia.