Home › Histology › Lymphocytes

Lymphocytes

Lymphocytus

For medical students2 min readUpdated 2026-10-10

Lymphocytes are the principal cells of the immune system belonging to the agranulocytic leukocyte group, responsible for the specific recognition and destruction of foreign antigens. Unlike other mature blood cells, they are capable of mitosis upon antigenic stimulation and function primarily outside the bloodstream.

Populations in bloodT cells — ~65%, B cells — ~30%, NK cells — 5–15%

StructureLarge hyperchromatic nucleus and a narrow basophilic cytoplasmic rim

Main functionSpecific immune response (humoral and cell-mediated)

Special propertyThe only mature leukocytes that retain the ability to divide

Morphology and General Characteristics

Lymphocytes, along with monocytes, are classified as agranulocytes. This means their nuclei are not segmented, and their cytoplasm is largely devoid of specific granules.

Based on size, circulating lymphocytes are divided into three groups:

Small lymphocytes (6–7 µm) — make up the vast majority (80–85%).
Medium lymphocytes (8–9 µm) — about 10%.
Large lymphocytes (12–15 µm) — account for 5–10%.

A typical small or medium lymphocyte features a round, intensely stained (hyperchromatic) nucleus that occupies almost the entire volume of the cell. The cytoplasm is represented by a narrow basophilic rim. The only morphological exception is natural killer (NK) cells, which appear as 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 main developmental lineages of B cells:

B1 lymphocytes (about 20% of blood B cells). Formed during the embryonic period. They mount 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 avian Bursa fabricii, in mammals they form in the red bone marrow. Their activation requires both contact with an antigen and a secondary co-stimulatory signal from T helper cells. This process yields highly specific plasmocytes and memory B cells.

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 cannot recognize free antigens. Their receptor (TCR) reacts only to antigenic determinants (epitopes) that have been processed and presented on the surface of other cells in association with special protein molecules.

Functional classification of T lymphocytes:

Cytotoxic T cells (T killers): Mediate cell-mediated immunity. Directly destroy foreign, tumor, or intracellularly infected cells by releasing perforin (which forms pores) and granzymes, as well as initiating Fas-dependent apoptosis.
T helper cells: Central immune regulators. Type 1 T helpers ($T_{H1}$) activate macrophages and cytotoxic T cells (cellular response, inflammation), whereas Type 2 T helpers ($T_{H2}$) assist in activating B lymphocytes for antibody production (humoral response).
Suppressor T cells: Suppress excessive immune responses (though this role may also be fulfilled by certain subsets of T helpers).
Delayed-type hypersensitivity (DTH) T cells: Mediate delayed-type hypersensitivity reactions (chronic inflammation) by recruiting macrophages and neutrophils to the site.

NK Cells (Natural Killers)

These are a specialized class of lymphocytes belonging to neither the T nor B lineage (so-called "null" cells). They originate in the red bone marrow and play a critical role in antitumor immunity.

Their primary feature is the absence of specific Ig-like receptors. NK cells are programmed to recognize self-proteins on normal cells. If these proteins are altered due to viral infection or malignant transformation, the NK cell is activated and destroys the target using perforin and granzymes.

Additionally, natural killer cells express receptors for the Fc fragment of IgG immunoglobulins, granting them the ability to attack cells already "tagged" with antibodies (antibody-dependent cellular cytotoxicity, ADCC).

Mnemonic

To remember name sources and functions: B lymphocytes (from Bursa) — Breed antibodies (humoral immunity); T lymphocytes (from Thymus) — Require direct cell contact (cell-mediated immunity).

Frequently asked questions

In complex with which surface molecules (proteins) on antigen-presenting cells do T lymphocytes recognize antigens?

T lymphocytes recognize antigens only when presented in complex with major histocompatibility complex (MHC) molecules:

MHC class I (MHC class I) — expressed on all nucleated cells and present endogenous antigenic determinants to cytotoxic T cells.
MHC class II (MHC class II) — expressed exclusively on professional antigen-presenting cells (macrophages, B lymphocytes, thymic epithelial reticular cells) and present exogenous antigenic determinants to T helper cells.

What differentiation antigens (CD markers) are characteristic of T helpers and cytotoxic T cells?

These T-lymphocyte subpopulations express specific coreceptors that serve as differentiation markers:

CD4 — marker for T helpers, which binds to the antigenic determinant–MHC class II complex.
CD8 — marker for cytotoxic T cells, which binds to the antigenic determinant–MHC class I complex.

Immature T cells express both coreceptors simultaneously prior to thymic selection.

Which classes of immunoglobulins (antibodies) can plasma cells secrete?

Plasma cells secrete antibodies of several distinct classes depending on the stage of the immune response:

Immunoglobulin M (IgM) — initially produced during the primary immune response following primary differentiation.
Immunoglobulin G, E, or A (IgG, IgE, IgA) — synthesized following genetic class-switch recombination. Notably, mature mucosal plasmocytes predominantly secrete IgA.

Can T and B lymphocytes be distinguished under a standard light microscope?

No, most T and B cells are morphologically indistinguishable, appearing as ordinary small or medium lymphocytes. They can only be differentiated using immunological methods, such as identifying specific surface membrane marker proteins.

Why are lymphocytes referred to as immunocompetent cells?

This is due to their ability to specifically recognize particular antigens via unique membrane receptors (BCR on B cells and TCR on T cells). The exception is NK cells, which operate via a different recognition mechanism.

What is the difference between a B-cell receptor and an antibody?

A B-cell receptor (BCR) is firmly anchored to the membrane of a mature B lymphocyte. Antibodies are protein molecules of identical specificity that are not membrane-bound, but are instead massively secreted by plasma cells into the blood and tissue fluid.

Go deeper

Molecular structure of antibodies: light and heavy chains, domain organization.
Immunoglobulin classes: characteristics of IgM, IgG, IgA, IgE, IgD and their tissue 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 site (ABS).