Myeloid cell populations
The immune system relies on strict cell specialization. Myeloid cells can be broadly divided into two major groups based on their localization:
- Circulating in the bloodstream. This group includes monocytes and three types of granulocytes: neutrophilic, eosinophilic, and basophilic.
- Tissue cells. These are populations residing permanently in organs and tissues. They include mast cells, macrophages, and dendritic cells (with the latter two forms developing from monocytes that have left the bloodstream).
The entire pool of myeloid cells shares a common approach to threat detection. They express a powerful receptor apparatus on their surface designed to recognize PAMPs (pathogen-associated molecular patterns). Dendritic cells play a crucial role, acting as the primary mediator that transmits information from the innate immune branch to the adaptive one.
Lymphocytes and specifics of antigen recognition
Unlike the myeloid lineage, lymphoid cells implement distinct, more refined recognition mechanisms. There are three main types of lymphocytes: T cells, B cells, and natural killer (NK) cells.
NK cells belong to the innate immune system. Their main difference from myeloid elements is that they recognize specific stress-induced molecules rather than PAMPs. In addition, NK cells lack clonal specificity, meaning they do not differ from one another in the spectrum of targets they recognize.
T and B lymphocytes, by contrast, purposefully recognize antigens using unique receptors:
- B lymphocytes use the BCR receptor. It is capable of binding native (unchanged) antigen in both free and membrane-bound forms.
- T lymphocytes bear the TCR receptor, which "sees" exclusively antigen fragments presented in a complex with MHC (major histocompatibility complex) molecules.
During the differentiation of T and B cells, complex genetic rearrangement takes place. As a result, each cell begins to express a receptor of absolute unique specificity, and all its progeny (clone) inherit this specificity. The body exerts rigorous selection, eliminating most dangerous autospecific clones. Due to this, immune responses involving these cells have a strict clonal type — only those lymphocytes whose receptors ideally match a specific foreign agent are recruited into the response.
Natural B-lymphocyte subpopulations
During normal maturation, independent of foreign antigen action, three basic B-cell subpopulations are formed:
- B1 cells. Based in serous cavities and barrier tissues. Their receptors have low specificity. The main task of this pool is the spontaneous production of low-affinity antibodies (primarily of the IgM isotype), which can also bind to autoantigens.
- Marginal zone B cells. In their properties, they are very similar to B1 lymphocytes, but are localized strictly in the marginal zone of the spleen.
- B2 cells ("conventional" B cells). These are classical participants in the humoral response. They populate the spleen, lymph nodes (including follicles), bone marrow, and intestinal lymphoid tissues. It is B2 cells that are responsible for synthesizing highly specific and high-affinity antibodies of various isotypes upon encountering an infection.
Diversity of T lymphocytes
The number of natural T-lymphocyte subpopulations significantly exceeds the diversity of B cells. The main pools are represented by γδ T and αβ T cells, which differ radically in receptor type (TCR), recognition specificity, and range of functions.
Of particular interest is the subpopulation of NKT lymphocytes, which stands out among classical αβT cells. These cells are unique in that they combine most of the functions of natural killer (NK) cells with certain properties of T lymphocytes. NKT lymphocytes differ from ordinary αβT cells in two key features:
- The repertoire of their receptor (TCR) specificities is severely restricted.
- They specialize in recognizing lipid epitopes, whereas classical T cells recognize peptide fragments.
Briefly: answers to topical questions
What classes of pattern recognition receptors (PRRs) do myeloid cells express?
Myeloid cells express a wide range of membrane and intracellular pattern recognition receptors. The main classes of these receptors include:
- Toll-like receptors (Toll-like receptors, TLR) — membrane and intracellular receptors.
- NOD-like receptors (Nucleotide oligomerizing domains, NLR) — intracellular receptors.
- Scavenger receptors (Scavenger receptors) — membrane receptors.
- Integrins — membrane receptors acting as coreceptors.
- C-type lectin receptors — membrane receptors (e.g., dectin-1).
It is TLRs and NLRs that act as key activation factors for myeloid cells in innate immune responses.
With which molecule do NKT lymphocytes recognize lipid antigens?
NKT lymphocytes recognize lipid and glycolipid antigens in a complex with the non-classical MHC-I-like molecule CD1d. Unlike conventional αβ T cells that recognize peptide epitopes, NKT cells specialize in lipid recognition. The CD1d molecule itself is synthesized in the endoplasmic reticulum, where it binds glycolipids for their subsequent presentation on the cell surface.
What surface CD markers are characteristic of NK cells?
Sources directly indicate the expression of the FcγRIII receptor (CD16) on NK cells. It is also noted that the IL-12 receptor is most prominently expressed on NK cells, incorporating the IL-12Rβ2 subunit (CD212). A complete list of surface CD markers for NK cells is not provided in the sources.
What receptors do NK cells use to assess the presence of MHC I and stress-induced molecules on a target cell?
To recognize stress-induced molecules on a target cell, natural killers use the activating receptor NKG2D. Its ligands are stress proteins — MIC (MHC class I-related chain) molecules and ULBP (UL-16 binding proteins) proteins.
Specific names of receptors assessing the presence of class I MHC molecules are not provided in the text. It is only indicated that NK cells are activated when a target cell loses the expression of classical MHC-I, while the presence of classical or non-classical (HLA-G or E) MHC-I molecules on its surface blocks killer activity.
To remember the difference between receptors: BCR (B cells) grabs the antigen Briefly and Bypassing intermediates (native), whereas TCR (T cells) Requires Precise presentation via MHC molecules.
Class and exam questions
How does recognition by NK cells differ from myeloid cells?
Myeloid cells respond to conservative microbial patterns (PAMPs), while NK cells recognize stress-induced molecules on the body's own cells.
What is the main functional difference between B1 and B2 lymphocytes?
B1 cells spontaneously produce low-affinity antibodies (IgM) in barrier tissues, whereas B2 cells ("conventional" ones) synthesize highly specific and high-affinity antibodies of various isotypes.
What are NKT lymphocytes and what do they recognize?
This is a special group of αβ T cells combining the properties of killers and T lymphocytes. Unlike conventional T cells, they recognize lipid epitopes rather than peptide ones.
What else to study on this topic
- Mechanisms of gene rearrangement encoding TCR and BCR antigen receptors
- The process of lymphocyte selection and elimination of autospecific clones
- The role of MHC (major histocompatibility complex) molecules in antigen presentation
- Structural and functional differences between γδ T and αβ T cells
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