Cancer is a result of genetic changes in the cells, which can also be a consequence of external influences. Some types of cancer cells are able to deactivate the cellular death program which normally ensures that cells die when they are no longer needed by the organism.

Capsule: Some bacteria have a capsule made of carbohydrates which protects them from phagocytosis. Bacteria of this type can cause extracellular infections. Phagocytes cannot ingest and destroy them until they are first bound by antibodies and attacked by components of the complement system which are produced as part of an acquired immune response.

CD4 T helper cells are CD4 T cells which support the B cells in their antibody reaction against an antigen.

Cellular immune response: The cellular immune response includes all adaptive immune reactions in which antigen-specific T cells play a key role. They include all parts of the adaptive immune system which cannot be transferred with the serum antibodies - the primary elements of the humoral immune response - to a non-immune recipient.

Cellular immunology is the study of the cellular basics of immunity.

Central lymphatic organs: The central lymphatic organs are where the lymphocytes develop. In humans, the B cells are formed in bone marrow, while the T cells form in the thymus gland from precursor cells which come from the bone marrow.

Clone: A clone is a population of cells which all stem from a common precursor cell.

Complement: A group of proteins required if certain antigen-antibody reactions are to take place is called a complement.

Complement receptors are surface proteins of various cells. They recognize and bind complement proteins, which themselves are bound to a pathogen. Complement receptors on phagocytes enable these cells to recognize and destroy pathogens enveloped by complement proteins.

Complement system: The complement system consists of a series of plasma proteins which together attack extracellular pathogens. It is spontaneously activated for some pathogens, in other cases by the binding of antibodies to the pathogen. The envelope of complement proteins which then surrounds the pathogen simplifies its destruction by phagocytes. The complement proteins alone can also kill the pathogen.

Congenital immunity: The early phases of a defence reaction come from congenital immunity; a wide range of congenital resistance mechanisms recognize a pathogen and react to its presence. Congenital immunity exists in all individuals and at all times; it does not increase, even if there is repeated contact with the pathogen, and does not distinguish among various pathogens. A defence reaction brought about by congenital immunity is normally followed by an acquired immunity response based on the clonal selection of specific lymphocytes and leading to long-term protection from a renewed infection.

Contact allergy: A contact allergy is a form of delayed hypersensitivity in which the T cells react to antigens which enter the body through the skin.

Co-receptor: A co-receptor is a cell surface protein which increases the sensitivity of an antigen receptor to its antigen by binding to neighbouring ligands. It participates in the signal cascade which leads to activation. CD4 and CD8 are examples of MHC-binding co-receptors on T cells.

Corticosteroids are steroids (hormones) such as cortisone formed in the adrenal glands. Corticosteroids can kill lymphocytes, especially maturing thymocytes, by triggering apoptosis. They are administered as medications to inhibit inflammation and as immunosuppressives.

Cortisone is a substance which inhibits inflammation.

Cross reaction: In a cross reaction, an antibody binds to an antigen which was not used in the production of the antibody. If an antibody which has been specifically produced against antigen A binds to antigen B, there is said to have been a cross reaction with antigen B. Generally speaking, this expression is used to describe the reaction of antibodies or T cells to antigens other than the ones that triggered the reaction.

Cross test: Cross tests are used for blood typing and histocompatibility tests to determine if a donor or recipient has antibodies against the cells of the other person which could cause problems during a transfusion or transplant.

Cyclosporin A is an effective immunosuppressive medication. It inhibits the signal transmission via the T cell receptors and thus prevents the activation of the T cells so that they cannot exercise their effector functions.

Cytokines are proteins from cells which influence the behaviour of other cells. Cytokines produced by lymphocytes are often called lymphokines or interleukins (abbreviated IL). Cytokines work through specific receptors on their target cells.

Cytotoxic T cells are T cells which can kill other cells. Most cytotoxic T cells are CD8 T cells aimed at MHC class I molecules, but CD4 T cells can also kill other cells in some cases. Cytotoxic T cells are important for defence against cytosolic pathogens.