Immunology

Humoral immunity is mediated by B cells and their secreted antibodies. B cells differentiate into plasma cells that produce antibodies which circulate in body fluids (blood, lymph) - the 'humor'. T cells are responsible for cell-mediated immunity, not humoral immunity.

T cell receptors (TCRs) recognize antigen only when presented by MHC molecules on antigen-presenting cells. Unlike antibodies, TCRs cannot bind free antigen. Options A, C, D are incorrect: TCRs are membrane proteins (not defined by glycosylation), they don't bind cytokines directly, and complement binding is not their function.

One of the most remarkable aspects of the human immune system is its diversity, i.e. the ability to recognise a wide range of antigens and to mount a specific antibody response. This is called clonal selection.

The CD3 complex is a multi-subunit protein complex normally expressed on T cells as part of the T cell receptor (TCR) complex. It's involved in signal transduction when the TCR binds antigen. CD3 is not expressed on B cells, granulocytes, or NK cells - these have their own marker molecules.

Plasmodium falciparum causes malignant tertian malaria (falciparum malaria), the most severe and life-threatening form. P. vivax causes vivax malaria (benign tertian), P. malariae causes quartan malaria, and P. ovale causes ovale malaria (mild tertian). Only P. falciparum is termed 'malignant' malaria.

Adaptive (acquired) immunity involves B cells, T cells, and antigen-presenting cells (APCs like dendritic cells, macrophages). B cells produce antibodies, T cells mediate cellular immunity and help B cells, and APCs process and present antigen to activate lymphocytes. All three cell types are essential components of the adaptive immune system.

The hypervariable regions in the variable domains of antibodies (produced by B cells) are directly responsible for antigen specificity. These complementarity-determining regions (CDRs) form the antigen-binding site. While T cell receptors and MHC molecules are involved in immune recognition, the antibody hypervariable regions are the primary source of specificity at the molecular level.

Contact dermatitis (e.g., poison ivy reaction, nickel allergy) is a classic Type IV hypersensitivity reaction, which is T-cell mediated and delayed (48-72 hours). The Arthus reaction is Type III (immune complex mediated), anaphylaxis requires previous sensitization (Type I), and the Mantoux test is also Type IV (delayed-type hypersensitivity).

Edible vaccines face challenges with dosage consistency due to variable plant growth conditions, expression levels, and fruit/vegetable size. The other options are actual advantages: therapeutic proteins are pathogen-free, plant tissues are heat-stable (no cold chain needed), and oral vaccines can work despite maternal antibodies (mucosal immunity).