Porcine lymphocytes are produced in large quantities in the primary lymphoid organs. B lymphocytes are produced in the bone marrow, and are responsible for the  humoral response. T lymphocytes are produced in the spleen, and are responsible for the cellular response. Some of these lymphocytes migrate to the secondary lymphoid organs where they form the T or B dependent areas . These cells are responsible for the three main characteristics of the immune system:

SELF RECOGNITION, SPECIFITY AND IMMUNOLOGICAL MEMORY.

 When using an ordinary microscope, no differences can be found (from a morphological point of view) between T and B lymphocyte. Both lymphocyte types are cells from 7 to 9 micrometers, with a voluminous nucleus and a little cytoplasm. Using scanning electron microscopy, remarkable differences can be observed.  T lymphocytes present a smooth and even surface while B lymphocytes have multiple projections, (these are the surface immunoglobulins).

 T and B lymphocytes by scanning electron microscopy

Giemsa stained lymphocyte, observed by ordinary microscopy

In the last decades, different surface markers to differentiate among lymphocyte populations have been designed. However, the development of monoclonal antibodies (MAb) against the different lymphocyte porcine cells have permitted the differentiation of these populations, as well as  their inclusion in different groups depending on their membrane antigens and functions. 

Porcine T lymphocytes forming rosettes with sheep erythrocytes.

B lymphocytes  of the pig are produced in the bone marrow, in a proportion of two hundred to four hundred million a day. This shows the huge capacity of response of the immune system. You must remember that each lymphocyte secretes a specific antibody (1 cell = 1 antibody type). In the peripheral  blood, B lymphocytes make up 8-18% of total lymphocytes. The lymphocyte membrane has a large number of molecules, many of them having being studied thanks to MAb. Among them it is important to underline the importance of B Cell Receptor complex or BcR complex. 

B lymphocyte

BcR consist of several chains. Some of them are variable (they are immunoglobulins), in which each lymphocyte presents different variations depending on the immunoglobulin type (mainly IgM and IgG, but also IgA or IgE) or on the antigen type. 

B lymphocyte BcR complex

The remaining two chains are constant (consisting of two chains, a and b), common to every B lymphocyte. The function of the variable chains, which in fact are immunoglobulins, is to interact with the specific antigen, while constant chains transmit the signal to the cell interior in order to initiate the antibody production. 

B lymphocytes  are thus characterized  by having immunoglobulins on their surface, mainly IgM and IgG. This allows their interaction with the antigen in its native form (T lymphocytes can not react with an antigen in its native form) (Chapter 3). The majority of the antigens that react with B lymphocytes are proteins, although they can also interact with polysaccharides. The initiation of the antigen reaction is always through the membrane immunoglobulins, (BcR signal), but antibody production needs, in most cases, the collaboration of CD4+ lymphocytes  (chapter 3). The antigens that need this collaboration are known as T- dependant antigens, and the majority of antigens belongs to this group. Other antigens, such as some bacteria polysaccharides or lypo polysaccharides, can produce antibodies without the collaboration of T lymphocytes. These are known as T- independent antigens. 

Using MAb, other B lymphocytes markers have been described. These surface molecules have been grouped in clusters of differentiation, on the basis of their antigenic and functional differences. 

Any stimulated B lymphocyte, either by a T- dependent antigen or by a T- independent antigen, generates a plasma cell clone, which will produce and secrete a large quantity of antibodies. These antibodies are specific for the epitope that induced the immune response. This response, mediated only by antibodies, is known as humoral response. The stimulated B lymphocyte, or plasma cell, has a morphology different from the original B lymphocyte (large nucleus and small cytosol) and more similar to a "factory- cell", with a small nucleus and a large cytosol. These cells are mainly located in lymph nodes, spleen red pulp, bone marrow and gut and respiratory mucosa. Their "in vivo" mean life is short: 2 or 3 days. It is even shorter  "In vitro, only a few hours. 

B lymphocyte stimulation, transformation in plasma cell, and antibody secretion.

Porcine T lymphocytes are produced in the spleen, and in smaller quantities in the T-dependent zones of secondary lymphoid organs. They differ from B lymphocytes in the sense that  they do not present surface immunoglobulins, but form rosettes with sheep erythrocytes.  T- lymphocytes play a main role in the immune response. In the first case, in the antigen-presenting mechanisms for B lymphocytes to produce antibodies, and in the second case, they are responsible for cellular immunity (immune response mediated by cells, not by antibodies) (Chapter 7).

Rosettes formation by T lymphocytes with sheep erythrocytes.

Receptor a-b (TcR a-b). They comprised 40 to 60% of the lymphocytes present in peripheral blood. T LYMPHOCYTES a b are: Helper T cells CD4+ and cytotoxic lymphocytes CD 8+
Receptor  g- d (TcR g-d). Present in most cells formerly known as null cells. Now they are known as T LYMPHOCYTES gd 

These receptors consist of two heavy chains and two light chains, similar to those of immunoglobulins. Their role is to interact with the antigens presented to them by antigen- presenting cells associated with SLA. 

Just as in the case of B lymphocytes, the production of MAb against porcine lymphocytes, has made it possible to differentiate several T cell subpopulations. They have been grouped in clusters of differentiation (CD) depending on their antigenic and functional differences. 

Thanks to these markers, it has been confirmed that pigs present some differences in respect to other animal species and humans.. Thus, the pig has a lymphocyte subpopulation which is double positive to CD4 and CD8 (CD4+CD8+). The percentage of this population increases with age. In this way, when the animal is one week old, this percentage is smaller than 2% of the total lymphocytes. When it is 3 years old, the percentage is 30%. The role of these double positive lymphocytes was thought to be related to memory cells. However, now it seems they are related to the helper activity of T lymphocytes during primary infections. It seems that there are two different populations of helper T cells, one being CD4+CD8- and the other CD4+CD8+. Both populations act as helpers during the primary response (at least, they do so "in vitro"). During the secondary response only CD4+CD8+ seem to have a role.It is also known that these double positive populations not only increase with age, but they are also located in large proportion in the tonsils (50%) and lymph nodes (30%) in adults. In regard to cytotoxic T lymphocytes, two different subpopulations of DC4-CD8+ can be found depending on the expression of CD6. Thus, lymphocytes expressing CD6 antigen are related to spontaneous cytotoxicity, while CD6+ SLA I are related to virus-infected cells cytotoxicity.