CTLs can kill cells infected with viruses or intracellular bacteria. They also can kill cancerous cells. How might an infected or cancerous cell evade immune control by CTLs? a. Increased expression of MHC class II molecules.
b. Increased expression of MHC class I molecules.
c. Decreased expression of MHC class I molecules.

The correct option is C (Decreased expression of MHC class I molecules.)

Explanation:

The lymphocyte is the major type of white blood cells which is divided into two:

--> B Lymphocyte and

--> T Lymphocyte

The T Lymphocyte is further divided into two namely:

--> T helper cells and

--> Cytotoxic T Lymphocyte (CTL)

The T lymphocyte cells recognize a non self target such as a pathogen only after pathogen has been processed and it's antigen presented in combination with a complex known as the Major histocompatibility Complex ( MHC). There are two types of MHC which includes Type 1 and Type 2 MHC.

The cytotoxic T Lymphocyte only recognize antigen combined with Class 1 MHC.

Some virus mechanism of action, while invading some target cells, is to reduce the normal levels of surface class 1 MHC in order to evade or escape the immune control by cytotoxic T Lymphocyte. Therefore the correct statement is decreased expression of MHC class I molecules.

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