<span>The immune system is the body's natural defense mechanism against organisms that can cause infection.There are three types of mechanisms:
1. Cellular :</span><span> refers to the recognition and/or killing of virus and virus-infected cells by leukocytes and the production of different soluble factors (cytokines) by these cells when stimulated by virus or virus-infected cells
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2. Structural: </span>barriers and the immune system defend the body<span> against organisms that can cause infection. </span>Natural<span> barriers include the skin, mucous membranes, tears, earwax, mucus, and stomach acid.
3. Chemical: </span><span>Some of these include the low pH of the stomach, which inhibits the growth of pathogens; blood proteins that bind and disrupt bacterial cell membranes; and the process of urination, which flushes pathogens from the urinary tract.
</span><span>antibodies - chemical
tissues lining hollow organs - cellular
phagocytes - cellular
skin - structural
antigen - chemical
leukocytes - cellular</span>
Answer:
They affect chemical reactions by lowering the activation energy, thus speeding up the reactions.
