Final answer:
Intracellular pathogens reproduce within host cells by avoiding the immune response, often leading to cell death. They have small genomes and can be obligate or facultative. CTLs and NK cells are key to the immune response by identifying and eliminating infected cells.
Step-by-step explanation:
Intracellular pathogens in the nucleus or cytosol invade host cells and reproduce within them, often resulting in cell death, such as lysis. These pathogens include viruses, bacteria, and parasites that can evade certain immune responses and exploit the nutrients within the host cell for their proliferation. Cytotoxic T cells (CTLs) and NK cells play crucial roles in identifying and destroying cells infected by intracellular pathogens. Many intracellular pathogens have a small genome since they rely on host cellular machinery for reproduction. Obligate intracellular pathogens can only reproduce within host cells, while facultative intracellular pathogens have the ability to reproduce both inside and outside of host cells.
They may employ various strategies to avoid the immune system, such as inhibiting phagosome-lysosome fusion within phagocytes or downregulating MHC class I molecules to evade detection by CTLs. When dealing with intracellular infections, CTLs target cells that present pathogen epitopes on MHC I molecules. Professional Antigen-presenting Cells express MHC class I molecules, presenting intracellular antigens to stimulate a CTL immune response. This response is essential for combating viruses, the most common class of intracellular pathogens.