1st ESO

3rd ESO

4th ESO

Biology 2nd Baccalaureate

Index by courses



Skip navigation

13.3.1. Components of the immune system

Components of the immune system

Molecules involved in the immune system


An antigen is any substance capable of triggering the immune response and inducing the formation of antibodies.

An antigen can be any protein or polysaccharide that is foreign to the body.

The epitope or antigenic determinant is a small portion of the antigen that is recognized by the antibody, to which it binds.


The antibodies or immunoglobulins are glycoproteins produced by B lymphocytes having a three dimensional structure that allows them to bind antigen by the binding site, called paratope.

Antibodies can act as surface antibodies, if they remain attached to the membrane of the B lymphocyte, or circulate through the blood or other body fluids, such as breast milk, tears, etc.

Major histocompatibility complex

The Major Histocompatibility Complex (MHC) is made up of a set of genes whose products are expressed on the surface of cells of the immune system. The immune system recognizes the glycoproteins and glycolipids of the plasma membrane of these cells as its own and this means that the immune response is not triggered, with the exception of autoimmune diseases.

The MHC was discovered for its involvement in the acceptance or rejection of organ and tissue transplants, which is why it was given that name.

The molecules synthesized from the MHC intervene in the development of specific immune responses, both humoral and cellular, since they intervene in the recognition of the antigen by T lymphocytes (helper, TH, and cytotoxic, TC).

Two types of major histocompatibility complex (MHC) are distinguished:

  • MHC-I: determine membrane glycoproteins that appear in almost all cells of the body, and serve to present peptide antigens from altered self cells (cancerous or virus-infected) to cytotoxic T lymphocytes (TC).
  • MHC-II: determine glycoproteins of antigen-presenting cell membranes (macrophagesdendritic cellsB lymphocytes), and serve to present exogenous peptide antigens (of the infectious agent) to T helper lymphocytes (TH).


The lymphocytes are a type of leukocytes (white blood cells) are found in blood and lymph. Its nucleus is large and rounded, and it has little cytoplasm. They cannot form pseudopods and therefore do not engulf. Its main function is to regulate the specific (or adaptive) immune response, reacting against foreign materials (microorganisms, tumor cells or antigens in general).

There are the following types of lymphocytes:

  • B lymphocytes. They develop and mature in the bone marrow and then migrate to different lymphatic tissues. They are responsible for the humoral immune response, transforming into plasma cells or plasma cells that produce antibodies (specific proteins) in the presence of antigens.
  • T lymphocytes. They are formed in the bone marrow and develops in thymus and involved in cellular immune response. They do not produce antibodies, but instead cause the death of certain cells that are altered and, therefore, unwanted. There are three types of T lymphocytes:
    • Helper T lymphocytes (TH). They recognize antigen-presenting cells and activate B lymphocytes
    • Cytotoxic T lymphocytes (TC). They recognize the antigens that are attached to the cells of the body, and destroy them, acting on cells infected by viruses or tumor cells.
    • Suppressor T lymphocytes (TS). They suppress the autoimmune response that can destroy cells in your own body, they act at the end of the immune response.

Lymphoid organs

The stem cells of lymphocytes, like those of other blood cells, are formed in the red bone marrow and are transformed into mature lymphocytes in the lymphoid organs. There are two types of lymphoid organs:

  • Primary lymphoid organs. In them the definitive maturation of the lymphocytes takes place and they are:
    • The bone marrow. The B lymphocytes they mature in the red bone marrow.
    • The scam. The T cells leave the bone marrow and mature in the thymus, gland located between the sternum and the trachea, and that increases in size until puberty, and then taper off.
  • Secondary lymphoid organs. Lymphocytes accumulate in these organs and there they undergo a final differentiation. Mature lymphocytes arrive and contact antigens to initiate the immune reaction.

The main secondary lymphoid organs are: the spleen, the lymph nodes (located along the lymphatic system, being more abundant in the groin, armpits, ...), the appendix, the intestinal peyer's patchestonsils and adenoids.

Questions that have come out in University entrance exams (Selectividad, EBAU, EvAU)

Murcia, July 2021, question 4.7

List the two main types of lymphocytes, the organs where they mature, and the type of immunity in which each of them participates. (1 point)


Legal warning






Follow us if it has been useful to you

Biology and Geology teaching materials for Compulsory Secondary Education (ESO) and Baccalaureate students.