The sequence of events taking place in the cell mediated response are outlined below. The first four are the same as in the humoral response.

1) Macrophages digest pathogen and present antigen on MHC II markers to specific TH cells.
2) Specific TH cells bind to MHC II complex via specific cell receptors.
3) TH cell is activated by interleukin I released by the macrophage
4) Activated TH cell proliferates and releases interleukin II.
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5) Interleukin II activates specific cytotoxic cells or killer T cells (TC cells).
6) Activated TC cells proliferate
7) TC cell binds to MHC-I antigen complexes on infected cells and releases chemicals that initiate apoptosis in target cells. TC cells also release perforins that degrade target cells by punching holes in the membrane.
TC cells only attack
- virus infected cells
- cancer cells
- cells of transplanted organs.
8) TC and TH memory cells are produced and persist for an extended period of time.

A cell known as the Natural Killer (NK) cell belongs to the group of cytotoxic T cells, however, they are not activated by the MHC-antigen interaction and are therefore not specific against any particular antigen. NK cells form part of the innate immune system which means that they are always active and do not rely on an antigen response to activate them. NK cells form an early line of defence against viruses and they do this by killing virally-infected cells. This is achieved by the release of two agents contained within granules in the NK cell.
- The first agent is called Perforin which forms pores in the virally-infected host cell.
- The second agent are called Granzymes which cause the apoptosis (programmed cell death) of the virally-infected host cell.

NK cells recognise the virally-infected cells due to the lack of MHC Class I on the surface of the cells. Viruses are capable of redirecting MHC Class I away from the surface of host cells so that it is not expressed. When MHC Class I is present on the surface of host cells it interacts with an inhibitory receptor on the NK cell to prevent the NK cell from releasing Perforin and Granzymes. When the MHC Class I is absent it cannot interact with the NK cells inhibitory receptor and so the NK cell is stimulated to release the Perforin and Granzymes.

View the video on the right and answer the questions below.

The cell mediated response is similar to the humoral response in that both rely on antibodies to neutralise antigens both rely on macrophage ingestion of the pathogen both generate long term memory cells and the proliferation of plasma cells

The Killer T cells are activated by antibodies antigens chemical released from activated Helper T cells antigens on the surface of the Helper T cell's membrane and ingesting the cell

Natural Killer cells are specific to unique antigens have an immunological memory are part of the innate immune system

Natural Killer cells do not target tumor cells virus particles cells from transplanted organs virus infected cells

Natural Killer cells kill by releasing antibodies by ingesting foreign cells on contact.

The Natural Killer cell undergoes apoptosis, programmed cell death, after it attacks a rogue cell. true false

Natural killer cells destroy neighbouring tumor cells by binding to specific antigens on the surface of the tumor cell. releasing antibodies. releasing chemical toxins