Lymphocytes

Lymphocytes 2 types: small and big ones. It is a morphological distinction and the most interesting thing is the immunological approach. - Small lymphocyte :7/8 micrometer in diameter. The nucleus occupies almost the entire cell which results in a high N/P ratio. The core ist round, the chromatin is dense and in MO no nucleolus is evident. The cytoplasm is reduced to a fine border. - Large lymphocyte :9 to 15 micrometers in diameter. The nucleus has slightly less condensed chromatin, the cytoplasm is pale, not very basophilic and sometimes we can see some azurophilic granulations. ❖ In ME :A small, barely outlined nucleolus is highlighted. There is little organelle intracytoplasmic therefore the small or large lymphocytes are resting cells. - Quantitative: 1 to 4 giga/L. <1 giga is lymphopenia. >4 giga is lymphocytosis. From a functional point of view we distinguish 2 populations:B and T lymphocytes. ➔ B lymphocytes intervene in humoral immunity essentially therefore mediated by antibodies. ➔ T lymphocytes are involved in cell-mediated immunity although in humoral mediation they are necessary for the proper synthesis of antibodies. ➔ Null lymphocytes :Some call themselves “NK” for natural Killer. These elements are capable of killing cells independently of the MHC. ➔ B lymphocytes represent 20% of blood lymphocytes. We can highlight on their surface Ig. When they are naive LBs who have not been in contact with antigens we highlight IgM, IgD. We have kappa or lambda Ig (1/3) and we have a preference for the kappa type at 2/3. Classically for B lymphocytes they carry CDs 19, 20 on their membrane, these are molecules called pan B, that is to say they will mark all the LBs. We also find the MHC class II molecules on their surface. These LB have their origin in the MOH where they carry out all their stages of maturation and the elements which leave the MOH are mature elements but which have not been in contact with antigens therefore they are LB naive. Some immature BL may present CD10. Why look for these differentiation antigens? For example, acute lymphoblastic leukemia which affects the BL will often express CD10, the patient is treated, he will be in remission, there will be disappearance of the leukemic cells and in the blood there are no cells expressing CD10 physiologically. ❖ B lymphocytes when there is an antigenic stimulus can change their morphology. In this diagram we have the stem cell at the level of the MOH, the precursor B, the LB, a contact with the antigen and the LB can become a centroblastic cell which will be found in the lymphoid follicles, it is a very basophilic cell with fine chromatin therefore morphology active, nucleoli in a peripheral position close to the nuclear membrane. This centroblast can become the centrocyte = cell morphologically less active and we no longer see the nucleolus but it is characterized by a nuclear notch and this centrocyte can give either memory B lymphocytes or plasma cells at the origin of antibody synthesis. On the other hand, the lymphocyte activated after antigenic stimulation can give a immunoblastic cell which is large, very basophilic, an average N/P ratio and the characteristic is that we see a large nucleolus in a central position. This immunoblast can give a plasma cell.

➔ T lymphocytes :represent 75% of circulating lymphocytes. No Ig will be found on their surface, we will find for example CD2 which is the first molecule described on T lymphocytes which is a receptor for sheep red blood cells. The CD3 is important, it is an antigen carried on all T lymphocytes and this CD3 is associated with the TCR which is the receptor for the antigens which will be presented to the L T. So a cell antigen presenter will express on its surface an antigen which is shaped in association with class II MHC molecules and the T lymphocyte recognizes this antigen through this TCR receptor which is coupled to CD3.