IgG provides systemic pathogen neutralization and can cross the placenta. It circulates in high amounts in the blood as a monomer, allowing it to diffuse through tissues, bind a wide range of pathogens, neutralize toxins, and opsonize microbes for phagocytosis. It also efficiently activates the classical complement pathway, enhancing bacterial clearance. Importantly, IgG is transported across the placenta via the neonatal Fc receptor (FcRn), delivering protective antibodies to the fetus. In contrast, IgA mainly protects mucosal surfaces as secreted dimers and does not cross the placenta; IgM is a large pentamer that is great at activating complement but largely remains in the bloodstream and also does not cross the placenta; IgD has limited secreted antibody and functions primarily as a B cell receptor.

Monocytes are produced in the bone marrow and circulate in the blood. When they migrate into tissues during infection or inflammation, they differentiate into macrophages, becoming large phagocytic cells that engulf microbes and debris and present antigens to T cells. This transformation is the defining step that links circulating monocytes to tissue macrophages. Lymphocytes are primarily involved in adaptive immunity, neutrophils are rapid first responders that phagocytose but do not differentiate into macrophages, and eosinophils deal with parasites and allergic responses rather than becoming macrophages.

The final stage of cellular immunity activation is clonal expansion: once a T cell that recognizes its antigen is activated, it proliferates to form a large clone of identical cells. These include effector cells that actively fight the current infection (like cytotoxic T cells and helper T cells) and memory T cells that persist for future encounters. This combination—rapidly expanding effector cells to deal with the present threat plus memory cells for faster responses later—best captures the end stage of T cell activation. The other statements describe earlier steps or incomplete outcomes. Activation begins with the T cell receptor recognizing antigen plus a co-stimulatory signal, which is not the final stage. Presenting the antigen to B cells relates to humoral immunity rather than the final T cell–driven response. Differentiation into memory cells occurs, but without the accompanying clonal expansion into effector and memory populations, it’s not the full final stage.

MHC Class II molecules are specialized for presenting exogenous, or extracellular, peptide fragments to CD4+ helper T cells, and they are expressed on professional antigen-presenting cells such as dendritic cells, macrophages, and B cells. These cells take up extracellular proteins by endocytosis, process them in acidified endosomes, and load the resulting peptides onto MHC II molecules. The peptide–MHC II complex then travels to the cell surface to be recognized by CD4+ T cells, which helps coordinate the immune response, including helping B cells and activating macrophages. This is distinct from MHC Class I, which is present on almost all nucleated cells and presents endogenous peptides to CD8+ cytotoxic T cells. Also, MHC II molecules are membrane-bound, not secreted; while B cells can present antigens via MHC II, the molecules themselves are not released into the surrounding environment.