The rest of the above tissues except A are peripheral lymphoid tissues. Thymus and bone marrow are central lymphoid tissues.

Humoral immunity, such as memory B cells and antibodies, are types of specific immunity (adaptive immunity).

B is incorrect. Chemokines are signaling proteins that attract leukocytes and guide their migration toward sites of infection or tissue injury; they do not physically carry macrophages. Histamine promotes inflammation, type I interferons induce an antiviral state, and lysozyme breaks down bacterial peptidoglycan.

Macrophage, dendritic cells, and neutrophiles are major phagocytic cells. Eosinophils are minor phagocytic cells. Basophils secrete histamine and anticoagulant heparin to damaged tissues or allergic reactions.

IgD, IgE, and IgG are monomeric. Typical secreted IgM is pentameric, whereas secretory IgA is mainly dimeric. Serum IgA is mainly monomeric, and membrane-bound IgM is monomeric. With the specified forms, A and B are correct, so E is the complete answer.

Plasma cells differentiate from activated B cells and synthesize and secrete antibodies.

A typical alpha-beta T cell receptor has one alpha chain and one beta chain linked by a disulfide bond. Its antigen-binding region recognizes peptide presented by MHC. The immunoglobulin portion of a B cell receptor has two heavy and two light chains; a fixed total of four disulfide bonds is not a universal property.

Helper T cells support B cell activation and antibody responses to protein antigens and can support effective CD8 T cell responses. Their loss can impair both arms of adaptive immunity. B cells and cytotoxic T cells do not develop from helper T cells, and not every B cell response requires T cell help.

Class II MHC on a macrophage presents peptide to a CD4 helper T cell. Class I MHC, expressed on nearly all nucleated cells, presents peptide to CD8 T cells. Perforin contributes to target-cell killing; antibodies are secreted by plasma cells.

The epitope is called an antigenic determinant, and is bound by antibodies. CD8 at cytotoxic cells binds class I MHC molecules. CD4 at helper T cells binds class II MHC molecules. Cytokines are proteins secreted by immune cells to activate lymphocytes.

B cells are maturated at bone marrow and T cells are maturated at the thymus. Spleen is the site for filtering or destruction of red blood cells. Lymph nodes are the place for filtering bacteria or foreign matter.

II, III, and IV are correct. An Rh-negative mother carrying an Rh-positive fetus may become sensitized to the D antigen through exposure to fetal red blood cells. Maternal anti-D IgG can cross the placenta and destroy fetal red blood cells. Rh inheritance affects fetal antigen status, but genetic dominance does not itself cause hemolysis. Incompatibility alone does not mean disease will occur.

Lymph nodes filter bacteria or foreign particles. Secretion of thymosin hormone happens in the thymus.

Histamine can increase local vascular permeability, allowing more plasma proteins to enter the injured tissue.
Leukocyte recruitment, hemostasis, pathogen clearance, and tissue repair involve additional signals and overlapping processes; they do not follow a single universal linear sequence.

The image shows pentameric IgM with a J chain. IgM is prominent early in a primary antibody response and activates complement, so B is correct. IgG is the most abundant antibody in blood and crosses the placenta. IgA supports mucosal protection, including through breast milk. Naive B cells commonly express membrane IgM and IgD; the pictured secreted pentamer is not a B cell receptor.

The immunoglobulin portion has two identical heavy chains and two identical light chains, so A is correct. These chains are linked by disulfide bonds, with additional intrachain bonds. Bond counts vary with immunoglobulin class and subclass. The complete B cell receptor complex also includes associated signaling proteins, which are not counted in these blanks.

A is false. Conventional cytotoxic T cells arise from the CD8 T cell lineage, not from helper T cells. Antigen recognition with appropriate activation signals can generate effector and memory CD8 T cells. Effector cytotoxic T cells can kill infected or tumor cells using perforin and granzymes.

I, III, and IV are true; II is false. Antibacterial antibiotics kill susceptible bacteria or inhibit their growth and do not treat viral infections. Some inhibit bacterial protein synthesis, while others act on targets such as cell-wall synthesis. Antifungal and antiprotozoal drugs are distinct antimicrobial categories; they should not be conflated with antibacterial antibiotics in this question.

Memory B cells retain receptors for the antigen that activated their lineage and can respond rapidly on re-exposure. Both memory B cells and plasma cells arise from B cells. Plasma cells secrete antibodies and may be short-lived or long-lived.

This recipient is blood group B. Group B and group O red blood cells lack A antigen, so D is correct. This question considers ABO red cell compatibility only; Rh type, other blood-group antibodies, and crossmatching must also be considered clinically.

C and D are correct. Natural infection and vaccination can stimulate the recipient’s own immune system and produce active immunity. Passive immunity is acquired by receiving preformed antibodies from another source. Interferons are signaling proteins involved in immune responses, including antiviral responses.

IgE, a monomer type, functions in allergic reactions.

Humoral immunity is mediated by B cells and their antibody-secreting plasma-cell descendants and includes memory B cells. Helper T cells can support B cell activation, but B cells do not derive from helper T cells. Antibodies are synthesized and secreted, not proliferated. Cytotoxic T cell killing is a cell-mediated immune function, so E is the best answer.

D is incorrect. B cells, T cells, and natural killer cells are types of lymphocytes that develop from hematopoietic progenitors originating in bone marrow; mature lymphocytes do not produce these separate lineages. Activated B cells can differentiate into antibody-secreting plasma cells. Cytotoxic T cells and NK cells can kill infected cells, and programmed cell death is called apoptosis.

C and D are incorrect, so E is the complete answer. Immunological memory generally supports a faster, larger, and more sustained response to the same antigen. Illness is not inevitable during either a primary or secondary response; prior immunity also does not guarantee that illness cannot occur. Activated B cells proliferate and can differentiate into plasma cells that secrete antibodies.