The thymus and bone marrow are primary (central) lymphoid organs. Adenoids, Peyer’s patches, and the spleen are secondary (peripheral) lymphoid tissues or organs. Lymphatic vessels transport lymph and immune cells but are not themselves classified as primary or secondary lymphoid organs. Therefore, the thymus (A) is the only correct answer.

Antibodies are components of humoral adaptive immunity. Macrophages, the skin barrier, inflammatory responses, and natural killer cells contribute to innate immunity.

All pairs A–D are correct. Histamine promotes vascular changes during inflammation. Chemokines guide leukocyte migration along chemical gradients. Type I interferons help induce antiviral defenses, and lysozyme hydrolyzes bonds in bacterial peptidoglycan.

Basophils release histamine and other mediators during allergic inflammation. Macrophages, dendritic cells, and neutrophils are important phagocytes. Eosinophils are best known for parasite defense and allergic responses and have limited phagocytic capacity.

IgD and IgE are monomers. Secretory IgA is usually a dimer, whereas secreted IgM is usually a pentamer. Serum IgA and membrane-bound IgM can be monomers, so the antibody form matters.

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

Most human T cells express an αβ receptor consisting of one α chain and one β chain; a minority express γδ receptors. The antigen-binding immunoglobulin unit of a B-cell receptor instead contains two heavy chains and two light chains. Its number of interchain disulfide bonds depends on the isotype.

CD4⁺ helper T cells provide cytokines and other signals that support many B-cell responses and help activate CD8⁺ T cells. They coordinate these responses rather than giving rise to B cells or cytotoxic T cells. Some immune responses can occur without helper T-cell support.

MHC class II presents peptides to CD4⁺ helper T cells and is expressed by professional antigen-presenting cells such as macrophages, dendritic cells, and B cells. MHC class I presents peptides to CD8⁺ T cells and occurs on most nucleated cells. Perforin helps cytotoxic cells kill targets; it is not an antigen-presenting molecule.

An epitope, or antigenic determinant, is the part of an antigen bound by an antibody. CD8 and CD4 are T-cell coreceptors that interact with MHC class I and class II molecules, respectively. Cytokines are signaling proteins that regulate immune-cell responses.

B cells mature in the bone marrow, whereas T cells mature in the thymus. The spleen filters blood and removes aged red blood cells. Lymph nodes filter lymph and help initiate immune responses.

Statements II, III, and IV are correct. An Rh-negative mother carrying an Rh-positive fetus may become sensitized after exposure to Rh-positive red cells. Maternal anti-D IgG can cross the placenta and damage fetal Rh-positive red cells. Genetic dominance of the Rh-positive trait is not the direct cause of this immune-mediated disease. The parental blood-type combination alone does not establish that disease will occur.

Lymph nodes filter lymph and support immune responses. Thymosin is associated with the thymus, not lymph nodes. Peyer’s patches are aggregates of lymphoid tissue in the small intestine.

Histamine increases vascular permeability, allowing plasma proteins to leave small vessels and enter injured tissue. Hemostasis, inflammation, and tissue repair overlap; they do not always follow one rigid sequence. The question focuses specifically on histamine-mediated protein entry.

The figure shows pentameric IgM with a J chain. IgM is usually the first antibody class secreted in a primary response and efficiently activates complement. Surface IgM on naive B cells is monomeric. IgG is the most abundant serum antibody and crosses the placenta; secretory IgA predominates in mucosal secretions and contributes to protection through breast milk.

An IgG1 monomer contains two heavy chains and two light chains, for four polypeptide chains. Two heavy–heavy and two heavy–light disulfide bonds give four interchain bonds. Additional intrachain disulfide bonds are not included in this count. Bond numbers vary among immunoglobulin isotypes.

A is incorrect. Conventional cytotoxic T cells develop from activated CD8⁺ T cells, not from CD4⁺ helper T cells. Helper T cells can support activation but are not their parent lineage. Cytotoxic T cells can kill infected or malignant cells using mechanisms that include perforin and granzymes.

Statements I, III, and IV are correct. Antibacterial antibiotics kill bacteria or inhibit their growth. Some inhibit bacterial protein synthesis, while others target processes such as cell-wall synthesis or DNA replication. Antibacterial antibiotics do not kill viruses.

Memory B cells retain antigen-specific receptors and can respond rapidly after re-exposure to the same antigen. They can differentiate into antibody-secreting plasma cells. Plasma cells include both short-lived and long-lived populations; they are not all short-lived.

In the standard ABO model, a type B recipient has anti-A but not anti-B antibodies. Type B and type O red cells lack A antigen, so both are ABO compatible. This question concerns red cells; plasma compatibility follows different rules.

C and D are correct. Active immunity develops through a person’s own immune response after infection or vaccination. Passive immunity results from receiving preformed antibodies. Vaccines stimulate active immunity without all being infectious agents. Interferons are signaling proteins involved in immune defenses.

IgE binds receptors on mast cells and basophils. Cross-linking of bound IgE by an allergen can trigger release of histamine and other mediators during immediate allergic reactions.

Humoral immunity involves B cells, memory B cells, plasma cells, and antibodies. Helper T cells support many B-cell responses but do not give rise to B cells. Plasma cells synthesize and secrete antibodies. Cytotoxic T cells act primarily in cell-mediated immunity.

A–D are correct, so none is an exception. Antibodies can neutralize extracellular viruses, while cytotoxic T cells and natural killer cells can kill infected cells. B cells, T cells, and natural killer cells are lymphocytes. Apoptosis is one form of programmed cell death.

C and D are incorrect. Immune memory generally produces a faster, larger, and more sustained response upon re-exposure. Clinical illness is not a defining or inevitable feature of either a primary or secondary response.