During antibody-mediated opsonization, phagocytic cells (like macrophages and neutrophils) bind to the Fc region of the antibody. This Fc region is the constant part of the antibody that sticks out after the Fab (antigen-binding) regions attach to the target (like a bacterium). Phagocytes have Fc receptors that specifically recognize and bind to this Fc portion, leading to phagocytosis of the tagged pathogen.

• Fc = "Fragment crystallizable" region = site for phagocyte binding

• Fab = "Fragment antigen-binding" = binds the antigen

• Variable region = part of Fab that specifically recognizes the antigen

• Disulfide bonds = hold antibody chains together, not used for phagocyte binding

✅ Correct choice: Fc

Which of the following statements is true? Group of answer choices In the secondary response to an antigen, higher levels of IgM will be detected in an individual’s serum. In the secondary response to an antigen, lower levels of IgM will be detected in an individual’s serum. The lag period following secondary exposure to an antigen is longer than the lag period following primary exposure. The lag period following secondary exposure to an antigen is shorter than the lag period following primary exposure

The correct answer is: The lag period following secondary exposure to an antigen is shorter than the lag period following primary exposure

Explanation:

• In a primary immune response, the body takes time (longer lag period) to recognize the antigen and activate the immune system.

• In a secondary immune response, memory B cells and memory T cells respond much faster, resulting in a shorter lag period and a stronger antibody response, mostly IgG.

Let’s review the options:

• ❌ Higher levels of IgM in secondary response – False; IgG is dominant in secondary responses.

• ✅ Shorter lag period in secondary response – True

• ❌ Lower levels of IgM in secondary response – Misleading; IgM is still present, but not the main antibody.

• ❌ Lag period longer in secondary – False

✅ Correct choice: The lag period following secondary exposure to an antigen is shorter than the lag period following primary exposure

NK Cells

Type of immunity:

✅ Innate immunity

Subtypes:

✅ No well-defined subtypes (though there are functional differences, NK cells are not divided into CD4/CD8 like T cells)

Works on:

✅ Virus-infected cells and tumor cells

Function(s):

✅ Kills abnormal cells (such as virus-infected or cancer cells) using cytotoxic granules; also secretes cytokines like IFN-γ

Receptors that this cell "sees" to induce function:

✅ MHC I (absence or downregulation of MHC I activates NK cells)

NK cells use activating and inhibitory receptors to detect stress signals and low MHC I expression.