How to write a step function, x(t-tau), within a differential equation

I am trying to solve and plot the following system of equations, however I am struggling with plotting the T(t-tau) and E(t-tau) functions in Matlab

I have been trying to use the heaviside function but the resulting graphs are not changing with variance in tau. Any help would be greatly appreciated, thank you!

function final_project_4()

r1 = 0.1; k1 = 0.0001; r2 = 0.03; alpha = 24; k2 = 0.00005; k3 = 0.000024;
u2 = 0.3; gamma = 0.3; c = 3.8; u1 = 0.03; epislon = 0.1; N = 275;

tau = 15;

TSpan = [0 250];
Y0 = [50 780];
options = odeset('RelTol', 1e-10,'AbsTol',[1e-10,1e-10]);

[t,y] = ode45(@model,TSpan,Y0,options);

plot(t,y)
title('tau = 15')
xlabel('time (days)')
ylabel('concentration')
legend('T(t)-cancer cells','E(t)-effector cells')

    function res = model(t,y)
        T = y(1);
        E = y(2);

        Ttau = T*heaviside(t-tau) ;
        Etau = E*heaviside(t-tau) ;

        dTdt = r1*T - k1*T*E;
        dEdt = r2*T + alpha - u1*E - k1*T*E + (1-epislon)*k1*Ttau*Etau;
        res = [dTdt;dEdt];
    end
end