The immune system tries to stop invasions by foreign agents by either killing cells directly or by

Directly The immune system is designed to defend the body against foreign or dangerous invaders. Such invaders include

Microorganisms (commonly called germs, such as bacteria, viruses, and fungi)

Parasites (such as worms)

Cancer cells

Transplanted organs and tissues

To defend the body against these invaders, the immune system must be able to distinguish between

What belongs in the body (self)

What does not (nonself or foreign) normal immune response consists of the following:

Recognizing a potentially harmful foreign antigen

Activating and mobilizing forces to defend against it

Attacking it

Controlling and ending the attack

If the immune system malfunctions and mistakes self for nonself, it may attack the body’s own tissues, causing an autoimmune disorder, such as rheumatoid arthritis, Hashimoto thyroiditis, or systemic lupus erythematosus (lupus).

Some immune cells kill it by phagositos. Some of them(T cells,B cells) kill them via producing antibodies