Innate vs Adaptive Immunity: B Cells, T Cells and Antibodies

Innate immunity is the fast, nonspecific first line of defense that responds the same way every time. Adaptive immunity is slower on first exposure, targets one specific antigen, and leaves memory, so a second exposure is faster and stronger. Most MCAT questions ask which arm a cell belongs to, or which signal links the two.

Side by side

Innate players

• Neutrophils are the most abundant white cells and the first phagocytes to arrive.
• Macrophages engulf pathogens and present antigen. Dendritic cells are the best antigen-presenting cells and carry antigen to lymph nodes.
• Natural killer (NK) cells kill cells that have lost MHC I, which many virus-infected and tumor cells do.
• Complement is a cascade of plasma proteins that opsonizes (tags) pathogens, recruits cells, and can lyse bacteria.
• Interferons are released by virus-infected cells and put neighbors into an antiviral state.

Adaptive players

B cells mature in bone marrow. Each makes one antibody specific to one antigen. After activation (usually with help from a T cell) they become plasma cells, which secrete large amounts of antibody, and memory B cells. This is humoral immunity, aimed at extracellular pathogens and toxins.

T cells mature in the thymus and are the cell-mediated arm.

• Helper T cells (CD4) recognize antigen on MHC II and secrete cytokines that activate B cells, cytotoxic T cells and macrophages. HIV destroys them.
• Cytotoxic T cells (CD8) recognize antigen on MHC I and kill infected or abnormal cells with perforin and granzymes.
• Regulatory T cells suppress the response and protect against autoimmunity.

MHC I is on every nucleated cell and shows internal (intracellular) peptides to CD8 cells. MHC II is only on antigen-presenting cells and shows engulfed peptides to CD4 cells. A mnemonic: CD4 x MHC II = 8, and CD8 x MHC I = 8.

Antibodies

An antibody has two heavy and two light chains in a Y shape. The variable region at the tips binds antigen, and the constant region (Fc) sets the class and signals to other cells. Antibodies neutralize, opsonize, and activate complement.

• IgM is the first made in a primary response (pentamer).
• IgG is the most abundant, appears later, and crosses the placenta.
• IgA is in secretions such as breast milk and mucus.
• IgE drives allergy and defense against parasites through mast cells.

Primary vs secondary response

The first exposure takes about 1 to 2 weeks to reach a peak, mostly IgM, with a low peak. A second exposure to the same antigen gives a rapid, larger response within days, mostly IgG, because memory cells are already present. Vaccines exploit this. Passive immunity (injected antibody, or maternal IgG and IgA) is immediate but temporary because no memory forms.

Common traps

• Mixing up MHC classes. I is every nucleated cell and CD8. II is antigen-presenting cells and CD4.
• Calling NK cells adaptive. They are innate and have no antigen-specific receptor.
• Expecting memory from innate cells. Memory is an adaptive feature.
• Forgetting that red blood cells have no MHC I (no nucleus).

One-minute recap

Innate: fast, broad, no memory (neutrophils, macrophages, NK cells, complement). Adaptive: specific, with memory. B cells make antibodies (humoral). CD4 helpers use MHC II, CD8 cytotoxic cells use MHC I. IgM first, IgG later and in memory response. Second exposure is faster and bigger.