Comprehensive Immunology and Serology: Mononuclear Phagocytes, Lymphocytes, and MHC Expression

The Mononuclear Phagocyte System and Mast Cell Function

The mononuclear phagocyte system ($MPS$) is a collective group of cells that perform various effector functions, contributing significantly to chronic inflammation and healing processes. Within the broader context of tissue-resident immune cells, mast cells are identified as granulocytes whose blood-borne precursors are not yet clearly defined. These cells differentiate primarily within the tissues and are typically found residing near small blood vessels. When activated, mast cells release several substances that directly influence vascular permeability, including heparin, histamine, and other factors. They play a vital role in wound healing, the elimination of parasites, and angiogenesis.

Angiogenesis is defined literally as the creation of new blood vessels. It is a critical process for the growth and sustenance of new tissues, ensuring they have an adequate blood supply. This formation of blood vessels is essential in various normal physiological situations. Specific instances where angiogenesis is both necessary and important include the repair of wounds and the development of the placenta during pregnancy. By facilitating the expansion of the vascular network, mast cells support the long-term maintenance and repair of the host environment.

Lymphoid Progenitors and Lymphocyte Development

The lymphoid progenitor is the stem cell precursor responsible for the production of lymphocytes, which include $T$ cells, $B$ cells, and $NK$ cells. The adaptive immune system is primarily comprised of two types of lymphocytes: $B$ cells and $T$ cells. Both of these lineages originate in the bone marrow. However, they differ in their site of maturation. While $B$ lymphocytes remain in the bone marrow to mature, $T$ lymphocytes must migrate to the thymus to undergo their maturation process. Upon completion of their maturation, both types of lymphocytes enter the bloodstream and subsequently migrate to peripheral lymphoid organs where they can encounter antigens.

B Lymphocytes: Differentiation, Functions, and Immunoglobulin Classes

$B$ cells serve as a critical component of adaptive immunity by differentiating into memory cells and plasma cells. Plasma cells are responsible for the secretion of antibodies, also known as immunoglobulins. Antibodies perform a variety of essential functions, such as neutralizing toxins and viruses and opsonizing microbes. Opsonization is a process by which antibody and complement proteins bind to the surface of a microbe to enhance its phagocytosis. It is important to note that while antibodies lock onto antigens, they do not kill the pathogens themselves; rather, they mark them for death, leaving the actual killing to other cells like phagocytes. Activated $B$ cells can also become memory $B$ cells, which allow the immune system to respond more rapidly if rechallenged by the same specific antigen.

Immunoglobulins belong to a large family of chemicals categorized into five main classes. Immunoglobulin G ($IgG$) is responsible for marking microbes so other immune cells can recognize and address them. Immunoglobulin M ($IgM$) specializes in the direct killing of bacteria. Immunoglobulin A ($IgA$) is found congregating in bodily fluids such as tears and saliva, where it protects the various gateways into the body. Immunoglobulin E ($IgE$) provides protection against parasites and is a primary mediator in allergic reactions. Finally, Immunoglobulin D ($IgD$) remains bound to the surface of $B$ lymphocytes to assist in the initiation of the immune response.

B Cell Receptor Specificity and Antigen Presentation

Each $B$ cell is highly specialized, producing only one specific type of antibody. For example, one $B$ cell may produce antibodies specifically against the bacteria that cause pneumonia, while another may recognize a common cold virus. The $B$ cell antigen receptor ($BCR$) is a membrane-bound form of the antibody that the cell will eventually secrete after it differentiates into a plasma cell. Because of this structural relationship, the $BCR$ is also referred to as membrane immunoglobulin ($mIg$). Additionally, $B$ lymphocytes can function as antigen-presenting cells ($APCs$) because they express both $MHC$ class $I$ and $MHC$ class $II$ molecules on their surface.

T Lymphocyte Classification and the Cluster of Differentiation System

$T$ cells represent an important and diverse group of lymphocytes that mature in the thymus, where they undergo rigorous positive and negative selection processes. They are classified based on their specific functions and the presence of surface molecules known as the "cluster of differentiation" ($CD$). These proteins are essential for the cells' functions and serve as markers to distinguish different types of $T$ cells. The most common varieties are $CD4^+$ $T$ cells, known as helper $T$ cells, and $CD8^+$ $T$ cells, known as cytotoxic or killer $T$ cells.

Unlike $B$ cells, $T$ cells cannot recognize soluble or free antigens. They can only recognize protein-based antigens that are bound to specific receptors. This recognition is mediated by the Major Histocompatibility Complex ($MHC$), also known as the Human Leukocyte Antigen ($HLA$). Recognition requires forming a complex between the $T$-cell receptors ($TCRs$), their co-receptors (such as $CD3$, $CD4$, or $CD8$), and the $MHC$-antigen complex. Specifically, $CD4^+$ $T$ cells recognize antigens bound to $MHC$ class $2$ molecules, while $CD8^+$ $T$ cells recognize antigens bound to $MHC$ class $1$ molecules. Both types of $T$ cells possess the $TCR$ and the $CD3$ co-receptor, but they differ in their specific additional co-receptors ($CD4$ versus $CD8$).

Helper T Cells ($CD4^+$) and Detailed Subsets

$CD4^+$ helper $T$ ($Th$) cells are effector cells for cell-mediated immunity. In their initial state, they are considered naive and must be activated to begin their immune functions. Activation occurs through interaction with professional antigen-presenting cells ($pro-APCs$), primarily dendritic cells found in lymph nodules or follicles. This interaction leads to the development of more antigen-specific $TCRs$ and the production of cytokines that initiate immune responses in other cells. There are three primary subtypes of $CD4^+$ $T$ cells: $TH1$, $TH2$, and $TH17$.

$TH1$ $CD4^+$ cells are vital for activating macrophages and fighting intracellular infections, such as $\text{M. Tuberculosis}$. They promote cell-mediated immunity by secreting Interferon-gamma ($IFN-\gamma$), which activates macrophages and increases their surface expression of $MHC$ class $2$ markers. They also secrete Tumor Necrosis Factor-alpha ($TNF-\alpha$) to stimulate dendritic cell migration. Activated macrophages then produce $IL-12$, which further increases the differentiation of $TH1$ cells, creating an amplification loop. $TH1$ cells are also responsible for inducing delayed-type hypersensitivity ($DTH$).

$TH2$ $CD4^+$ cells are important for combating extracellular infections, particularly helminthic (parasitic) infections. These cells produce cytokines such as $IL-4$, $IL-5$, and $IL-13$, which activate and expand mast cells and eosinophils to clear parasites. They also activate macrophages to clear cellular debris and inflammation caused by large parasites. $TH2$ cells promote humoral responses, specifically the secretion of $IgE$, and play a significant role in allergic diseases. $TH17$ $CD4^+$ cells are vital for mucosal immunity and combating extracellular bacteria and fungi. They produce $IL-17A$, $IL-17F$, and $IL-22$, cytokines that activate neutrophils and monocytes and drive increased inflammation.

Cytotoxic T Cells ($CD8^+$) and Mechanisms of Killing

Once activated, $CD8^+$ $T$ cells (cytotoxic $T$ lymphocytes) migrate through the circulation to find antigenic targets, such as virally infected cells or cells containing intracellular bacteria. These cells are the primary effector cells of adaptive immunity and can cross the blood-brain barrier. They utilize two main mechanisms to kill their targets. The first involves the $Fas/Fas$ Ligand ($FasL$) pathway. Activated $CD8^+$ $T$ cells express $FasL$, which binds to the $Fas$ receptor ($CD95$) on the target cell, activating caspases and leading to apoptosis. This is common in treating cells infected with bacteria like $\text{Listeria}$ species or viruses.

The second killing mechanism involves the release of granzymes and perforin. These compounds bypass cell walls to activate caspases directly within the target cell. Cytotoxic $T$ cells are activated by cytokines and can also attach to and destroy cancer cells. Their ability to migrate through blood vessel walls and non-lymphoid tissues makes them highly effective at patrolling the body for infected or malignant cells.

Regulatory T Cells and Natural Killer Cells

Regulatory $T$ cells, or suppressor $T$ cells, modulate immune responses and maintain tolerance to self-antigens, thereby inhibiting autoimmune processes. They produce inhibitory cytokines such as $IL-10$ and $IL-35$ and utilize $CTLA-4$ (Cytotoxic $T$-Lymphocyte Associated protein 4) to inhibit the $B7$ molecule on $APCs$, which decreases the immune response. The production of regulatory $T$ cells is spurred by $TGF-\beta$ and $IL-2$. In many autoimmune diseases, $IL-2$ is notably absent. These cells express $TCR$ ($CD3$), $CD4$, $CTLA-4$, and $CD25$, sharing a common lineage with helper $T$ cells.

Natural Killer ($NK$) cells represent a third lineage of lymphoid cells, though they may develop from both myeloid and lymphoid progenitors. Produced in the bone marrow, they are found in relatively low quantities in the bloodstream and tissues. $NK$ cells are cytotoxic and contain granules with perforins and granzymes. Despite lacking antigen-specific receptors, they can recognize and kill abnormal cells, including tumor cells and virus-infected cells. They are a core part of the innate immune defense against intracellular pathogens. When $TH1$ cells secrete $IL-2$ and $IFN-\gamma$, $NK$ cells are activated to become Lymphokine Activated Killer ($LAK$) cells.

The Major Histocompatibility Complex (MHC) and Antigen Presentation

The Major Histocompatibility Complex ($MHC$) consists of membrane proteins whose extracellular domains form a cleft where peptide fragments are bound and presented to $T$ cells. In humans, these are called Human Leukocyte Antigen ($HLA$) genes and are clustered on chromosome 6. Three specific genes ($HLA-A$, $HLA-B$, and $HLA-C$) encode $MHC$ class $I$ proteins. The $MHC$ class $II$ proteins are determined by several $HLA-D$ loci, specifically $DP$, $DQ$, and $DR$. Because $MHC$ proteins differ among individuals of the same species, they are classified as alloantigens.

The process of antigen presentation involves several steps within a host cell. First, a protein antigen enters the cell and is catabolized or "processed" into linear peptide fragments of varying lengths. Second, certain peptides bind selectively to $MHC$ molecules inside the cell. Third, the resulting $MHC$-peptide complex moves to the cell surface. Finally, the complex is recognized by a $T$ cell expressing the appropriate $TCR$. This critical role where $MHC$ molecules bind and present processed antigens is known as $MHC$ restriction. $CD8^+$ $T$ cell responses are restricted by $MHC$ class $I$, which are expressed on all nucleated cells. $CD4^+$ $T$ cell responses are restricted by $MHC$ class $II$, which are constitutively expressed only on $APCs$ (dendritic cells, macrophages, and $B$ lymphocytes) and thymic epithelial cells.

Endogenous and Exogenous Antigen Pathways

Antigen activation follows two distinct pathways depending on the source of the antigen. In the endogenous pathway, antigens originating within the cell are proteolyzed by the immune proteasome. Specific peptides are loaded onto $MHC$ class $I$ molecules and transported to the cell surface to be recognized by $CD8^+$ $T$ cells. This leads to the proliferation of cytotoxic $T$ cells that can kill any host cell presenting that specific antigen. This interaction also involves $NK$ cells; their interaction with $MHC$ class $I$ prevents them from killing normal, healthy host cells.

In the exogenous pathway, the antigen is taken up by $APCs$ via endocytosis. These antigens are broken down by lysosomes into peptides that are loaded onto $MHC$ class $II$ molecules. The complex is then transported to the cell surface to activate $CD4^+$ cells. These activated helper cells can then interact with $B$ cells presenting the same antigenic peptide on $MHC$ class $II$, leading the $B$ cell to differentiate into antibody-secreting plasma cells and memory $B$ cells. In the absence of specific inducing factors, most specialized tissue cells (like those in the liver or kidney) express $MHC$ class $I$ but not class $II$, whereas professional $APCs$ constitutively express both.