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Major histocompatibility complex

The major histocompatibility complex is a set of special proteins that combine with antigens from digested pathogens in macrophages and then display them on the outer surface membrane of the cell to produce and antigen-presenting cell.

Cell-mediated system

The response of the immune system to body cells that have changed in some way e.g. have been infected by viruses or have a mutation such as a cancer cell.

Humoral response

The response of the immune system to antigens found on the outside of pathogens or body cells.

Clonal selection

The process by which the B cells will make the right antibody to inactivate or destroy a particular pathogen are selected and then cloned to make large numbers of B effector cells and B memory cells.

B lymphocytes

Lymphocytes made in the bone marrow that are found both in the lymph glands of the body and free in the blood once they mature.

T lymphocytes

Lymphocytes made in the bone marrow that mature and become active in the thymus gland before moving into the blood


Protein molecules attached to cells that only bind to specific molecules with a particular structure


Cell-signalling molecules that activate cells in both the humoral and the cell-mediated immune responses.


A change in the arrangement or amount of genetic material in a cell.


Muscles or glands which bring about changes in response to a stimulus


An organism that is genetically identical to its parent.

The specific immune system: humoral response

The cells of your body carry a unique set of antigens on the cell surface membranes that mark them out as your cells. Some of these antigens will be common to every other member of the human species, others will be unique to you as an individual. Your antigens will usually be more similar to those of your relatives than to those of total strangers. The only organisms with totally matching antigens are identical twins and clones.

Your immune system enables your body to recognise anything that is non-self and remove it from the body as efficiently as possible. It responds to specific foreign cells and it is very diverse, recognising around 10 million different antigens. Your immune system also provides you with an immunological memory which means once your body responds to a pathogen the first time, it can respond rapidly if you meet it again.

The immune system is very complex. Part of the system responds to antigens found on the outside of pathogens or body cells. This is the humoral response and it is brought about mainly by the B lymphocytes. On the other hand, the cell-mediated system responds to body cells that have changed in some way – for example cells infected by viruses, or cells such as cancer cells that have a mutation. It is brought about mainly by the T lymphocytes.

B cells T cells
B effector cells Divide to form plasma cell clones T helper cells When activated by antigen presenting cells (APCs) produce cytokines that stimulate B cells and T cells
Plasma cell clones Produce antibodies to a specific antigen very rapidly (2000 molecules per second) T killer cells Lymphocytes that when activated produce chemicals that destroy pathogens or cells infected by pathogens
B memory cells Long-lived cells that help provide immunological memory so if the pathogen is encountered again they will rapidly produce the right antibody T memory cells Long-lived cells that help provide immunological memory so if the same pathogen is encountered again they form a clone of active killer T cells that destroy the pathogen

Both parts of the immune system depend on the activation of the T helper cells to produce special chemicals known as cytokines.

Immune system

The immune system is very complex – this is a much simplified model of how it works

The humoral response

The humoral response to antigens results in the production of antibodies, glycoproteins that are specific to a particular antigen. Antibodies are not attached to cells but are free to travel round the body in the blood and tissue fluid.

T helper cell activation
  • Pathogens enter the body and produce chemicals that attract phagocytes. Macrophages engulf and digest the pathogens then separate off the antigens. The antigens are combined with special proteins called the major histocompatibility complex (MHC). The antigen/MHC complexes move to the surface of the macrophage cell outer membrane to form an antigen-presenting cell (APC).
  • T cells with the right receptors on their outer membrane bind to the specific antigen/MHC complex on the APC, triggering the T cell to reproduce and form a clone of cells.
  • Most become active T helper cells. They produce cytokines that are important in the rest of the immune system. The remainder form inactive T memory cells, which remain in the body and rapidly become active if the same antigen is encountered again.
T helper cell activation

T helper cell activation

The effector stage

  • When a pathogen invades the body, some B lymphocytes will have immunoglobulins on their surfaces that match the antigens on the pathogen. The B cells bind to it and engulf it by endocytosis. As with the macrophage in T cell activation, fragments of processed antigen are attached to MHC proteins within the cell, and the MHC/antigen complex is transported to the cell surface membrane where the antigen is displayed.
  • An activated T helper cell (see above) binds to the specific antigen displayed on the MHC complex on the B cell, triggering the release of cytokines from the T helper cell.
  • The cytokines stimulate clonal selection, when the B cells divide and form clones of B effector and B memory cells are produced. The B effector cells differentiate to form plasma cells (plasma cell clones).
  • Plasma cells produce antibodies that are identical to the immunoglobulin of the original parent B cell at a rate of around 2000 molecules a second! Plasma cells do not live long but the antibodies they produce remain in the blood for varying lengths of time. B memory cells may stay in the blood for years.
Effector stage of the humoral response of the specific immune system

The effector stage of the humoral response of the specific immune system

The humoral response