Unlike humoral immunity, cell-mediated immunity is mediated by the cells, especially T cells of immune system. When a pathogen enters the body, the antigen presenting cells phagocytise the pathogen and processes the antigenic molecules to present the same to T cells through MHC class II complex.
As soon as TH cell interacts with antigen and MHC class II molecule complex of antigen presenting cell, it gets activated and starts secreting cytokines.
Cytokines released from T cells activates B cells, TC cells and various other phagocytic cells. Activated phagocytic cells are capable of killing microorganisms such as bacteria, protozoa etc. more efficiently.
Activated T cells differentiate into cytotoxic T lymphocytes (CTLs) and proliferate under the influence of cytokines produced by activated TH cells. CTLs kill all the cells displaying foreign antigens associated with MHC class I molecules.
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Since intra cellular pathogens express their characteristic antigens on the surface of its host cell, CTLs identify altered cells of their own as foreign agents through their new surface antigens and destroy them along with the pathogen.
Ever since tumor cells and cells of graft from a donor express new antigens on their surface, they are considered as foreign cells and are removed form the body by the CTLs.
Hence cell mediated immunity has a vital role in the removal of graft, intra cellular pathogens and tumor cells.
Both humoral and cell mediated responses produced by immune system following its first encounter with an antigen are different from the subsequent exposures to the same antigen.
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The immune response generated following first encounter is known as “primary response” and the response for subsequent exposures is known as “secondary response”. The characteristic features of both primary and secondary immune responses are different.
Generally the secondary response is faster than the primary response due to the presence of memory cells.