Another main component of adaptive immunity is cell-mediated immunity. In addition, immune memory, which “remembers” a target after an infection, is the greatest feature of adaptive immunity.
Basic Knowledge
In cell-mediated immunity, killer T cells(cytotoxic T cells) play the main role.
① Dendritic cells and macrophages perform antigen presentation and activate helper T cells.
② Helper T cells activate killer T cells.
③ Killer T cells directly attack and destroy virus-infected cells, cancer cells, and transplanted cells.
④ Macrophages also carry out powerful phagocytosis as activated macrophages.
It is effective against pathogens hiding inside cells(such as viruses and tuberculosis bacteria) and abnormal cells, which antibodies cannot handle.
When the immune response ends, some of the B cells and T cells involved remain in the body for a long time as memory cells. When the same antigen enters again, the phenomenon in which memory cells respond more strongly and quickly than in the primary response is called the secondary response. This is immune memory.
Cell-mediated immunity（Immunity in which T cells directly attack cells）
Killer T cells（Cytotoxic lymphocytes that destroy infected cells）
Activated macrophages（Macrophages in a state with strong phagocytic activity）
Memory cells（Long-lived B and T cells that remember past antigens）
Primary response（The response to an antigen for the first time; slow and weak）
Secondary response（The second and later response by memory cells; fast and strong）
Rejection reaction（A reaction in which transplanted tissue from another person is attacked as foreign）
Further Study
Cell-mediated immunity also helps eliminate cancer cells. Tasuku Honjo of Japan discovered PD-1, a molecule that suppresses excessive attacks by T cells. By inhibiting it, he helped develop immune checkpoint inhibitors(Opdivo), which restore anticancer activity, and received the Nobel Prize in 2018.
The difference between primary and secondary responses is obvious when shown on a graph. If time is on the horizontal axis and antibody amount is on the vertical axis, antibodies increase slowly several days after the first antigen invasion and reach a low peak, whereas after the second invasion they rise rapidly to a peak more than 10 times higher within several days. The rejection reaction that is a problem in organ transplantation is caused by cell-mediated immunity through killer T cells, so it must be suppressed with immunosuppressants during transplantation.
- Cell-mediated immunity=killer T cells・activated macrophages attack
- Targets=virus-infected cells・cancer cells・transplanted tissue
- Memory cells carry out the secondary response
- Secondary response=fast・strong(more than 10 times the primary response)
- Rejection reaction=attack on transplanted tissue by cell-mediated immunity
- Immunosuppressants suppress rejection reactions
- Tasuku Honjo's discovery of PD-1→immune checkpoint inhibitors
Points to Note
① Distinguish the main players in humoral immunity(antibodies) and cell-mediated immunity(killer T cells). ② Helper T cells serve as commanders in both types of immunity. Killer T cells are specific to cell-mediated immunity. ③ Memory cells exist among both B cells and T cells. ④ The primary response takes 1〜2 weeks to reach its peak, while the secondary response peaks within several days; their scales differ greatly.
Practice
- What are the T cells that directly attack virus-infected cells called?
- Why does the secondary response occur more quickly and strongly than the primary response?
- Which T cells are the main agents of the rejection reaction that occurs during organ transplantation?