<P> Unlike animals, plants lack phagocytic cells, but many plant immune responses involve systemic chemical signals that are sent through a plant . Individual plant cells respond to molecules associated with pathogens known as Pathogen - associated molecular patterns or PAMPs . When a part of a plant becomes infected, the plant produces a localized hypersensitive response, whereby cells at the site of infection undergo rapid apoptosis to prevent the spread of the disease to other parts of the plant . Systemic acquired resistance (SAR) is a type of defensive response used by plants that renders the entire plant resistant to a particular infectious agent . RNA silencing mechanisms are particularly important in this systemic response as they can block virus replication . </P> <P> Another important role of the immune system is to identify and eliminate tumors . This is called immune surveillance . The transformed cells of tumors express antigens that are not found on normal cells . To the immune system, these antigens appear foreign, and their presence causes immune cells to attack the transformed tumor cells . The antigens expressed by tumors have several sources; some are derived from oncogenic viruses like human papillomavirus, which causes cervical cancer, while others are the organism's own proteins that occur at low levels in normal cells but reach high levels in tumor cells . One example is an enzyme called tyrosinase that, when expressed at high levels, transforms certain skin cells (e.g. melanocytes) into tumors called melanomas . A third possible source of tumor antigens are proteins normally important for regulating cell growth and survival, that commonly mutate into cancer inducing molecules called oncogenes . </P> <P> The main response of the immune system to tumors is to destroy the abnormal cells using killer T cells, sometimes with the assistance of helper T cells . Tumor antigens are presented on MHC class I molecules in a similar way to viral antigens . This allows killer T cells to recognize the tumor cell as abnormal . NK cells also kill tumorous cells in a similar way, especially if the tumor cells have fewer MHC class I molecules on their surface than normal; this is a common phenomenon with tumors . Sometimes antibodies are generated against tumor cells allowing for their destruction by the complement system . </P> <P> Clearly, some tumors evade the immune system and go on to become cancers . Tumor cells often have a reduced number of MHC class I molecules on their surface, thus avoiding detection by killer T cells . Some tumor cells also release products that inhibit the immune response; for example by secreting the cytokine TGF - β, which suppresses the activity of macrophages and lymphocytes . In addition, immunological tolerance may develop against tumor antigens, so the immune system no longer attacks the tumor cells . </P>

Describe the various types of immune system and the immune cells involved in each type