The immune system has developed in all vertebrates as a protection against infectious diseases. Invertebrates possess only a primitive defense system and must rely on phagocytic cells (macrophages and neutrophilic cells). Although such cells also have an important function in protecting vertebrate organisms against infections, they are only one component of a much more complex and refined defense strategy, the immune system. Responsible for putting it together are the lymph-associated tissues and the T- and B-lymphocyte populations that mature there.

One distinguishes between two kinds of immune responses:

  • a cell-mediated immune response, for which the T- lymphocytes are responsible
  • a humoral immune response of the B-lymphocytes and plasma cells, which acts by producing antibodies.

Like the nerve system, the immune system has the ability "to remember". For this reason we develop, for example, after our first contact with the appropriate virus a lifelong immunity against the diseases caused by these viruses.

Two types of differentiated cells are responsible for the defense and memory:

  • Memory cells
  • Effector cells
Fig. 17 - Primary and secondary immune responses

Primary immune response
Secondary immune response
First contact with antigen A
Second contact with antigen A

Fig. 17

The secondary immune response after a second contact with antigen A is faster and stronger than the first one. In the lower diagram it is shown that the immunocompetent B or T cells proliferate and differentiate either to memory or effector cells when stimulated by antigen A.
After the second contact the latency for forming effector/memory cells is considerably shorter and the response is much stronger than with the first one.

Despite many similarities, the two lymphocyte populations, the B- and the T cells, differ in several fundamental aspects:

  1. Distance of the effect
  2. Type of antigen recognition
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Purely visually, one cannot tell resting B- and T-lymphocytes apart. For diagnostic purposes, however, the two populations of lymphocytes can be distinguished based on their plasma membrane proteins. For this, an antibody against the thy-1-glycoprotein that is present on the T cells is used. With the same immunohistochemical technique the maturity of cells can also be assessed, because according to how mature the cell is, various surface proteins are produced (see: differentiation of the receptor on T-lymphocytes in the thymus (interactive diagram) and differentiation of the receptors on B-lymphocytes in bone marrow (interactive diagram).

By stimulating B or T cells the difference is also visible. The B cell, which due to the stimulation has transformed into a plasma cell, is completely full of rER, and appears larger. The T cell remains the same size and possesses little rER; however, it has many free ribosomes.