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).

Chemokines induce chemotaxis: immune cells move along chemical gradients toward sites of injury or infection. Chemokines do not physically transport macrophages. Histamine promotes inflammation, interferons participate in antiviral responses, and lysozyme breaks down bacterial cell-wall peptidoglycan. Thus B is the incorrect pair.

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

IgD and IgE are typically monomeric. IgG is also monomeric but is not among the individual options. Secreted IgM is predominantly pentameric, whereas secretory IgA is predominantly dimeric. Serum IgA can also exist as a monomer, and membrane-bound IgM is monomeric. The forms specified in C and D exclude those monomeric exceptions, so E is the complete answer.

Plasma cells are differentiated B cells that produce and secrete large quantities of antibodies.

A typical αβ T-cell receptor has one α chain and one β chain linked by a disulfide bond. Its associated CD3 proteins transmit signals. The immunoglobulin component of a B-cell receptor has two heavy chains and two light chains. Disulfide bonds link heavy and light chains and the heavy chains to one another; the precise number varies with immunoglobulin structure.

CD4⁺ helper T cells can support B-cell activation, antibody production, and CD8⁺ T-cell activation and expansion. Their deficiency can impair several adaptive immune responses. Some B-cell and CD8⁺ T-cell responses can occur without helper T-cell support, so this support is not an absolute requirement in every response.

MHC class II presents antigenic peptides to CD4⁺ helper T cells. Macrophages also express MHC class I, which presents peptides to CD8⁺ T cells. The question specifically asks about CD4⁺ T-cell recognition, so B is the unique answer.

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 mature in the bone marrow, and T cells mature in 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.

Rh hemolytic disease of the fetus and newborn may develop when an RhD-negative mother becomes sensitized to RhD-positive fetal red blood cells and produces anti-D IgG antibodies. These antibodies can cross the placenta and destroy fetal red blood cells. Rh incompatibility alone does not guarantee disease. Statement I is false; II, III, and IV are true, so D is correct.

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

Histamine can increase local blood flow and vascular permeability, helping plasma antimicrobial proteins and immune cells reach injured tissue. Pathogen clearance and tissue repair follow. A best matches this simplified model. In actual inflammation, hemostasis, mediator release, cell recruitment, and repair overlap; they do not follow one mandatory sequence.

The figure shows pentameric secreted IgM with a J chain. IgM is prominent early in a primary immune response and efficiently activates complement. IgG is the most abundant serum antibody and crosses the placenta to provide passive fetal immunity. Secretory IgA protects mucosal surfaces and provides protection through breast milk. Membrane-bound IgM and IgD are monomeric B-cell antigen receptors; the figure shows secreted pentameric IgM.

The antigen-binding immunoglobulin component of a typical B-cell receptor has four polypeptide chains: two heavy chains and two light chains. Each light chain is linked to a heavy chain, and the heavy chains are linked to one another by disulfide bonds. The exact number of disulfide bonds varies among immunoglobulins. Associated Igα/Igβ proteins provide signaling and are not included in the four immunoglobulin chains.

Cytotoxic T lymphocytes differentiate from activated CD8⁺ T cells. CD4⁺ helper T cells may support their activation and expansion but do not directly differentiate into these CD8⁺ cytotoxic T cells. They mediate cellular immunity and can kill infected or cancerous cells using perforin and granzymes. A is false; B–D are true.

Antibacterial antibiotics kill bacteria or inhibit their growth. Different classes act through different mechanisms, including inhibition of cell-wall synthesis, bacterial protein synthesis, or nucleic-acid processes. They do not treat viral infection itself. I, III, and IV are true; II is false, so D is correct.

Memory B cells are long-lived B cells with antigen-specific receptors. On re-exposure to the corresponding antigen, they can reactivate, expand, and differentiate into antibody-secreting plasma cells. Plasma cells are effector B cells; some are short-lived and others are long-lived. Memory B cells do not arise from T cells.

A group B recipient has anti-A antibodies and can receive ABO-compatible red blood cells from group B or group O donors. This question concerns red blood cell transfusion, not plasma transfusion. Rh compatibility and other clinically relevant antibodies require separate assessment and crossmatching, so D is the complete ABO answer.

Natural infection and vaccination can induce active immunity. Passive immunity results from receiving preformed antibodies. Interferons are signaling proteins involved in immune responses. A and B are false; C and D are true, so E is the complete answer.

IgE mediates immediate (type I) hypersensitivity. Allergen cross-linking of IgE bound to mast cells or basophils can trigger release of histamine and other mediators. IgE is monomeric.

Humoral immunity is mediated primarily by B cells and the antibodies secreted by plasma cells. Helper T cells can support B-cell activation. Cytotoxic T cells are primarily involved in cell-mediated immunity. Thus E is the unique answer.

B cells, T cells, and natural killer cells are lymphocyte populations arising from hematopoietic progenitor cells. Antibodies secreted by plasma cells can neutralize viruses, cytotoxic T cells can kill infected cells, and natural killer cells can also kill infected cells. Apoptosis is programmed cell death. A–D are correct, so E is the appropriate answer.

The secondary immune response is generally faster and stronger than the primary response because of immunological memory, and it may be more prolonged. Re-exposure does not necessarily result in symptomatic illness. C and D are false, so E is the complete answer.