Morphology and general characteristics
Lymphocytes, along with monocytes, belong to agranulocytes. This means that their nucleus is not segmented, and there is practically no specific granularity in the cytoplasm.
According to their size, cells circulating in the blood are divided into three groups:
- Small lymphocytes (6–7 µm) account for the absolute majority (80–85%).
- Medium lymphocytes (8–9 µm) account for about 10%.
- Large lymphocytes (12–15 µm) account for 5–10%.
A typical small or medium lymphocyte has a rounded, intensely stained (hyperchromatic) nucleus that occupies almost the entire volume of the cell. The cytoplasm is represented by a narrow basophilic rim. The only exception in terms of appearance is natural killer cells (NK cells), which morphologically look like large granular lymphocytes.
B lymphocytes and humoral immunity
The B cell population is responsible for the humoral immune response. Upon recognizing an antigen, they transform into plasma cells (plasmocytes), which secrete protective proteins—antibodies (immunoglobulins, Ig)—into the extracellular environment.
There are two lines of B cell development:
- B1 lymphocytes (about 20% of blood B cells). Formed as early as the embryonic period. Provide a rapid response (within 48 hours) via direct transformation into plasmocytes, but their antibodies have low specificity.
- B2 lymphocytes (the main population). Named after the bursa of Fabricius in birds (Bursa fabricii); in mammals, they are formed in the red bone marrow. Their activation requires not only contact with an antigen but also an additional signal from T helper cells. As a result, highly specific plasmocytes and B memory cells are formed.
Secreted antibodies bind to antigens, leading to their destruction via phagocytosis, lysis (through complement system activation), or basophil degranulation.
T lymphocytes: cellular immunity and regulation
This group of cells matures in the thymus. Unlike B cells, T lymphocytes are unable to recognize free antigens. Their receptor (TCR) reacts only to antigenic determinants (epitopes) previously processed and presented on the surface of other cells in a complex with special proteins.
Functional classification of T lymphocytes:
- T-killers (cytotoxic cells). Responsible for cell-mediated immunity. Directly destroy foreign, tumor, or intracellularly infected cells. They utilize a mechanism involving perforin release (forming pores) and granzymes, as well as Fas-dependent apoptosis induction.
- T helpers. Central regulators of immunity. Type 1 T helpers ($T_{H1}$) activate macrophages and T killers (cellular response, inflammation), while type 2 T helpers ($T_{H2}$) help activate B lymphocytes to produce antibodies (humoral response).
- T suppressors. Inhibit excessive immune responses (although it is hypothesized that T helpers themselves may play this role).
- DTH T cells. Responsible for delayed-type hypersensitivity reactions (chronic inflammation) by recruiting macrophages and neutrophils to the focus.
NK cells (Natural Killers)
This is a special class of lymphocytes that belongs to neither the T nor the B lineage (so-called "null" cells). They are formed in the red bone marrow and play a crucial role in antitumor immunity.
Their main feature is the absence of specific Ig-like receptors. NK cells are tuned to recognize normal cells' own proteins. If these proteins are altered due to a viral infection or tumor transformation, the NK cell is activated and destroys the target using perforin and granzymes.
In addition, natural killers have receptors for the Fc fragment of IgG class immunoglobulins. This gives them the ability to attack cells that have already been "marked" by antibodies (antibody-dependent cellular cytotoxicity).
Briefly: answers to topical questions
In complex with which molecules (proteins) on the surface of antigen-presenting cells are T lymphocytes able to recognize antigens?
T lymphocytes are able to recognize antigens only in complex with major histocompatibility complex proteins.
- MHC class I (MHC class I) are present on all cells and present antigenic determinants to T killers.
- MHC class II (MHC class II) are present only on antigen-presenting cells (macrophages, B lymphocytes, thymic epithelioid reticulocytes) and present antigenic determinants to T helpers.
Which differentiation antigens (CD markers) are characteristic of T helpers and T killers?
These T lymphocyte subpopulations are characterized by specific co-receptors acting as differentiation markers.
- CD4 is a marker of T helpers that binds to the antigenic determinant and MHC-II complex.
- CD8 is a marker of T killers that binds to the antigenic determinant and MHC-I complex.
Immature T cells carry both of these co-receptors simultaneously on their membrane prior to selection.
Which classes of immunoglobulins (antibodies) can plasma cells secrete?
Plasma cells can secrete antibodies of several specific classes depending on the stage of the immune response.
- Class M immunoglobulins (IgM) begin to be produced during the primary response after initial differentiation.
- Class G, E, or A immunoglobulins (IgG, IgE, IgA) begin to be synthesized after genetic rearrangement and class switching. In particular, mature mucosal plasmocytes secrete immunoglobulin A.
Remember the name sources and functions: B lymphocytes (from Bursa) — Produce antibodies (humoral immunity); T lymphocytes (from Thymus) — Require direct cell contact (cellular immunity).
Class and exam questions
Can T and B lymphocytes be distinguished under a regular light microscope?
No, most T and B cells are morphologically indistinguishable and look like regular small or medium lymphocytes. They can only be separated by immunological methods, identifying specific marker proteins on the cell membrane.
Why are lymphocytes called immunocompetent cells?
This is due to their ability to specifically recognize particular antigens thanks to unique membrane receptors (BCR in B cells and TCR in T cells). The exception is NK cells, which operate on a different principle.
What is the difference between a B receptor and an antibody?
The B-cell receptor (BCR) is firmly anchored to the membrane of a mature B lymphocyte. Antibodies are protein molecules identical in specificity that are not fixed to the cell, but massively secreted by plasmocytes into the blood and tissue fluid.
What else to study on this topic
- Molecular structure of antibodies: light and heavy chains, domain organization.
- Classes of immunoglobulins: features of IgM, IgG, IgA, IgE, IgD and their localization.
- Stages of intracellular antigen processing and presentation.
- Molecular mechanisms of cytotoxicity: osmotic shock and Fas-dependent apoptosis.
- Clonal organization of B lymphocytes and the structure of the antigen-binding center (ABC).
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