Lymphocytes

CD4+ T cell

CD8+ T cell

B cell

Lymphocytes are the cells of adaptive immunity. They represent 20 to 40% of blood leukocytes but 99% of lymph leukocytes. We distinguish between T cells and B cells. These two categories of lymphocytes have many common features but also absolutely major differences.

Hematopoiesis and cytology

T and B cells originate from the same lymphoid progenitor and share the same morphological characteristics of small cells with round nuclei with a high nuclear-cytoplasmic ratio in the basal state. Microscopic observation does not allow T cells to be distinguished from B cells; this requires the use of flow cytometry techniques and clusters of differentiation (CD).

Maturation and selection

T and B cells go through several stages of maturation, notably structured around the progressive acquisition of the TCR for the former, and BCR for the latter.

Once their immunoreceptor has been acquired, the different cell clones go through positive and negative selection stages, guaranteeing both the conservation of “useful” clones, and the elimination of clones with auto-reactive potential (see T cell ontogeny / B cell ontogeny).

Adaptive immunity

Unlike innate immunity cells,adaptive immunity cells only become operational after a maturation process involving specific environments, actors and temporalities. They are said to be naive until they encounter the antigen for the first time. They can only be effectors after a priming step, unlike innate immunity cells.

Overall, the sequence can be summarised as:

Where:

• The effector cells carry the specific functions of fighting the antigen (primary response).
• The memory cells ensure the maintenance of the specific rearrangement of the antigen receptor allowing a faster and greater response during reinfection (secondary response).

V(D)J recombination

One of the main characteristics of lymphocytes is the acquisition of a specific receptor after somatic recombination : the TCR for the T cell and the BCR for the B cell. Each lymphocyte expresses in very multiple copies a single type of TCR or BCR receptor on its surface, the one produced by the V(D)J somatic rearrangement.

This mechanism makes it possible to generate a multitude of different immunoreceptors, to respond to the polymorphism of the potential antigens to be recognised.

Repertoire and memory

After the activation process, the lymphocytes have the particularity of expanding to form a cell clone. All the cells in the clone then have the same TCR or BCR receptor on their surface.

The set of cell clones that emerged through antigenic encounters constitutes the immune repertoire of an individual. There is great inter-individual variability resulting from genetic factors and environmental exposures.

Among cell clones, certain cells have the property of maintaining themselves over time and thus forming an immune memory, responsible for a more rapid response in the event of re-exposure to the antigen

There are major differences between the T cells and the B cells, listed in the table below and detailed on the actors and T cell activation / B cell activation pages

T cells

B cell