Lesson · B.5.2

The immune system

How the innate and adaptive immune systems defend the body, the cells each one relies on, and why HESI A2 Biology questions hinge on telling the two apart.

ObjectiveB.5.2Read4 minExam weightMicrobiology and Immunity ≈ 16% of Biology

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What you will be able to do

Tell the innate immune system apart from the adaptive immune system and say what each one responds to.
Name the main phagocytes and other leukocytes involved in an inflammatory response and what each one does.
Explain the difference between humoral immunity and cell-mediated immunity, and what memory cells give you.

What the lesson concludes

The innate system is fast and non-specific, physical barriers plus phagocytes; the adaptive system is slower but targets a specific antigen.
Neutrophils arrive first at a site of infection, macrophages follow and continue the cleanup.
B cells drive humoral immunity by secreting antibodies; T cells drive cell-mediated immunity by acting directly on infected cells.
Helper T cells activate other immune cells; cytotoxic T cells kill infected cells directly.
Memory B and T cells are why a second exposure to the same pathogen produces a faster, stronger response.